AGRICULTURAL

METEOROLOGY

Dr. S.M. Sureshkumar Dr. G. Baradhan

Dr. S. Manimaran Dr. N. Ramesh

Department of Agronomy

1

Faculty of AgricultureAnnamalai University

Imprint

2

FOREWORD

3

PREFACE

4

ACKNOWLEDGEMENTS

5

CONTENTS

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Chapter -1

Introduction to Agricultural Meteorology

Agricultural Meteorology – definition of

meteorology, agro – climatology- weather and climate,

factors affecting weather and climate – climate types –

scope of Agricultural meteorology, co-ordinates of India

and Tamil Nadu – Atmosphere – stratification and

composition.

Chapter – 2

Solar Radiation and Temperature

Solar radiation – light and heat energy – Intensity,

Quality, day length and direction of light, effect on crop

production, measurement – Temperature effect on crop

growth, cardinal temperature, diurnal and season

variations, isotherm – soil temperature and effect on

crop growth – measurement of temperature

Chapter - 3

Atmospheric pressure and wind systems of the

world

Atmosphere pressure – pressure system – causes for

variation, importance, isobar, measurement –

Atmospheric humidity – Absolute and specific humidity,

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vapour pressure, effect on crop growth , measurement.

Wind systems of world, tropical convergence zones,

speed and direction of wind measurement – effect on

crops.

Chapter – 4

Rainfall and Evaporation

Clods, classification and characteristics – precipitation –

forms – Hydrological cycle – rainfall – monsoons of

India, onset and withdrawal, effect on crop production –

measurement of rainfall. Isohyet – artificial rain making

– evaporation and transpiration, factors affecting

evaporation.

Chapter – 5

Weather Aberrations and forecasting

Weather aberrations – deficient rainfall (drought),

cyclone, anticyclones, hurricane, tornado, storms –

effect on agriculture – phenology and bioclimatic law,

Agricultural seasons of India – Agroclimatic zones of

world, India and Tamil Nadu – weather forecasting –

importance. Types – Agrimet observatory – synoptic

chart, crop weather calendar – remote sensing and crop

weather modelling – agroclimatic normals for field

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crops – effect of climate and weather on crop

production and pest and diseases.

Appendix I

Appendix II

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Chapter 1

INTRODUCTION TO AGRICULTURAL

METEOROLOGY AND ATMOSPHERE

Meteorology

The word meteorology was derived from a Greek

word “meteoros” means ‘above the earth’s surface’

which is nothing but atmosphere. The meaning of

“logy” is a study of science. Therefore, meteorology is

defined as a branch of science dealing with that of the

atmosphere. Meteorology is concerned with the

physical, chemical and dynamical state of the earth’s

atmosphere and it’s interaction with the underlying

surface.

Meteorology is essentially known as

observational science dealing with different

atmosphere variables such as pressure, temperature,

wind, humidity, clouds, and radiation at a given

moment. Meteorology predicts the behavior of the

atmosphere over a period of a few hours to a few days

ahead. The main task of meteorology is to provide

weather forecasting and communicate the same in time

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to the users. Another important duty of the

meteorology is to increase our knowledge of the

atmosphere through research and aiming at the

accuracy of weather forecast. Meteorology is a

combination of both physics and geography.

Meteorology is the science of weather. It is

essentially an inter-disciplinary science because the

atmosphere, land and ocean constitute an integrated

system. The three basic aspects of meteorology are

observation, understanding and prediction of weather.

There are many kinds of routine meteorological

observations. Some of them are made with simple

instruments like the thermometer for measuring

temperature or the anemometer for recording wind

speed. Countries around the world exchange the

weather observations through fast telecommunications

channels. These are plotted on weather charts and

analyzed by professional meteorologists at forecasting

centers. Weather forecasts are then made with the help

of modern computers and supercomputers. Weather

information and forecasts are of vital importance to

many activities like agriculture, aviation, shipping,

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fisheries, tourism, defence, industrial projects, water

management and disaster mitigation. Recent advances

in satellite and computer technology have led to

significant progress in meteorology.

Agricultural meteorology

In simple terms, agricultural meteorology is the

application of meteorological information and data for

the enhancement of crop yields and reduction of crop

losses because of adverse weather. This has linkages

with forestry, horticulture and animal husbandry. The

agro meteorologist requires not only a sound

knowledge of meteorology, but also of agronomy, plant

physiology and plant and animal pathology, in addition

to common agricultural practices. This branch of

meteorology is of particular relevance to India because

of the high dependence of our agriculture on monsoon

rainfall which has its own vagaries. So, Agricultural

meteorology can also be defined as follows:

A branch of applied meteorology which

investigates the physical conditions of the

environment of growing plants or animal

organisms

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An applied science which deals with the

relationship between weather/climatic conditions

and agricultural production

A science concerned with the application of

meteorology to the measurement and analysis of

the physical environment in agricultural systems.

The word ‘Agro meteorology’ is the abbreviated

form of agricultural meteorology

To study the interaction between meteorological

and hydrological factors on the one hand and

agriculture in the widest sense, including

horticulture, animal husbandry and forestry on

the other (WMO, Geneva)

Importance to crop production

1. Helps in planning the cropping patterns/systems

2. Selection of sowing dates for optimum crop

yields

3. Cost effective ploughing, harrowing, weeding etc

4. Reducing losses of applied chemicals and

fertilizers

5. Judicious irrigation to crops

6. Efficient harvesting of all crops

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7. Reducing or eliminating outbreak of pests and

diseases

8. Efficient management of soils which are formed

out of weather action

9. Managing weather abnormalities like cyclones,

heavy rainfall, floods, drought etc. This can be

achieved by

(a) Protection: When rain is forecast avoid irrigation.

But, when frost is forecast apply irrigation

(b) Avoidance: Avoid fertilizer and chemical sprays

when rain is forecast

(c) Mitigation: Use shelter belts against cold and

heat waves

10. Effective environmental protection

11. Avoiding or minimizing losses due to forest fires.

Future scope of agricultural meteorology

1. To study climatic resources of a given area for

effective crop planning.

2. To evolve weather based effective farm

operations

3. To study crop weather relationships for all

important crops and forecast crop yields based on

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agro climatic and spectral indices using remote

sensing

4. To study the relationship between weather

factors and incidence of pests and diseases of

various crops

5. To delineate climatic/agro ecological/agro

climatic zones for defining agro climatic analogues

so as to make effective and fast transfer of

technology for improving crop yields

6. To prepare crop weather diagrams and crop

weather calendars

7. To develop crop growth simulation models for

assessing/obtaining potential yields in different

agro climatic zones

8. To monitor agricultural droughts on crop-wise for

effective drought management

9. To develop weather based agro advisories to

sustain crop production utilizing various types of

weather forecast and seasonal climate forecast

10. To investigate microclimatic aspects of crop

canopy in order to modify them for increased crop

growth

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11. To study the influence of weather on soil

environment on which the crop is grown

12. To investigate the influence of weather in

protected environment (eg. Glass houses) for

improving their design aiming at increasing crop

production.

Divisions of Meteorology

Sl.N

o.

Division

name

Application

1. Dynamic meteorology

Deals with the forces that create and maintain motion and the latest transformations associated therewith

2. Physical meteorology

Deals with physical nature such as radiation, heat, evaporation, condensation and precipitation

3. Climatology Study of weather patterns over time and space

4. Synoptic meteorology

Deals with analysis and forecasting of the weather phenomena

5. Aeronautical meteorology

Application of meteorology to the problem of aviation

6. Maritime meteorology

Deals with marine navigation

7. Agricultural meteorology

Application of meteorology in relation to crop production

8. Hydrometeorology

Deals with meteorological problems related to water

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supply, flood control, irrigation etc.

9. Medical meteorology

Influence of weather and climate on the human body

10. Aerology Deals with the conditions of the free atmosphere on the basis of direct observations

Coordinates of India and Tamil nadu

Coordinates of India

Lies between 8º N and 37º N latitude

68ºE and 97º E longitude

Coordinates of Tamil Nadu

Lies between 08º 05’ N and 13º 35’ N latitude

76º 15’ E and 80º 20’ E longitude

The Earth is elliptical in shape and has three spheres

Hydrosphere - the water portion

Lithosphere - the solid portion

Atmosphere - the gaseous portion

Atmosphere

The word atmosphere derives from the Greek

word “Atmos” which means vapour and “Sphaira”

which means sphere (layers).The atmosphere is defined

as the colourless, odourless and tasteless physical

mixture of gases which surrounds the earth on all sides.

It is mobile, compressible and expandable. It contains

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huge number of solid and liquid particles called aerosol.

Atmosphere is a huge envelope of mixture of gases

extending several hundreds of kilometers from earth

surface. Some gases are permanent atmospheric

constituents in fixed proportions to the total gas

volume and others vary from place to place and time to

time.

Uses of atmosphere

1. Provides oxygen which is useful for respiration in

crops

2. Provides carbon-dioxide to build biomass in

photosynthesis

3. Provides nitrogen which is essential for plant growth

4. Acts as a medium for transportation of pollen

5. Protects crops plants on earth from harmful UV rays

6. Maintains warmth to plant life and

7. Provides rain to field crops as it is a source of water

vapour, cloud, etc.

Composition of atmosphere (Fig.1)

The following all the different gases that are present in percentage by volume approximately.Nitrogen (N2) = 78.08 Oxygen (O2) = 20.95Argon (Ar) = 0.93 Carbon di oxideCO2 = 0.03Neon (Ne) = 0.0018 Helium(He) = 0.0005Ozone(O3) = 0.00004 Hydrogen(H2) = 0.00006

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Methane (CH4) = 0.00017

Fig.1

Fig.2 Vertical Layers of atmosphere based on

temperature/Stratification (Fig.2)

On the basis of vertical temperature variation the

atmosphere is divided into different spheres or layers.

A) Troposphere

1. The troposphere is the

atmospheric layer closest to the planet and contains

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the largest percentage (around 80%) of the mass of

the total atmosphere.

2. The word “Tropo” means mixing

or turbulence and “sphere” means region.

3. The troposphere is

approximately 12 kilometers thick, but there are

slight variations. The average height of this lower

most layer of the atmosphere is about 14 km above

the mean sea level; at the equator it is 16 km and 7-

8 km at the poles.

4. All weather phenomenon that we

are primarily interested like various types of clouds,

thunderstorms, cyclone and anticyclones occurs in

the troposphere because of the concentration of

almost all the water vapour and aerosols in it. So,

this layer is called as “seat of weather phenomena”.

5. The wind velocities increase with

height and attain the maximum at the top of this

layer.

7. Another important feature of the troposphere is that

as height increases, temperature decreases. The

temperature drops about 6.5 ºC for every kilometer

above the earth's surface (Lapse rate). 

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8. Most of the radiation received from the sun is

absorbed by the earth’s

surface. So, the troposphere is heated from below.

9. In this layer, about 75 per cent of total gases and

most of the

moisture and dust particles present.

10. The top of the troposphere there is a shallow layer

separating it from

stratosphere which is known as the “Tropopause “.

11. The upper boundary of the layer Tropopause,

ranges in height from 5 miles (8 km) near the

poles up to 11 miles (18 km) above the equator. Its

height also varies with the seasons; highest in the

summer and lowest in the winter.

B). Stratosphere

1. The stratosphere is the second major strata of air in

the atmosphere. It extends above the tropopause

(around 20 km onwards) and extends to an altitude

of about 30 miles (50 km) above the planet's

surface.

2. The air temperature in the stratosphere remains

relatively constant up to an altitude of 15 miles (25

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km). Then it increases gradually with height until it

reaches about 10°C at an altitude of 50 km. 

3. Solar energy is converted to kinetic energy when

ozone molecules absorb ultraviolet radiation,

resulting in heating of the stratosphere.

4. This layer is called as “Seat of photochemical

reactions”.

5. Since the stratosphere is not turbulent this is where

most planes like to fly.

6. Less convection takes place in the stratosphere

because it is warm at the top and cold at the

bottom. 

7. There is also persistence of circulation patterns and

high wind speeds.

8. The boundary between the stratosphere and the

next layer mesosphere is called the stratopause.

C). Mesosphere/Ozonosphere

1. The layer between 50 and 80 km is called as

“Mesosphere”. In this layer the temperature

decreases with height.

2. There is a maximum concentration of ozone

between 30 and 60 km above the surface of the

earth and this layer is known as the ozonosphere.

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3. A property of the ozone is that it absorbs UV

rays. Had there been no layer of the ozone in the

atmosphere, the ultraviolet rays might have

reached the surface of the earth and no life can

exist.

4. The temperature drops in this layer to about -100

ºC. This is the coldest region of the atmosphere.

5. The stratosphere and mesosphere together are

sometimes referred to as the middle atmosphere.

Because of the preponderance of chemical process

this sphere is called as the “chemosphere”

6. The upper boundary of this layer is called the

“Mesopause”.

D). Thermosphere (Ionosphere)

1. The thermosphere layer lies beyond the

ozonosphere (mesosphere) at a height of about 80

km above the earth’s surface and extends up to 400

km. The air is very thin.

2. Thermosphere means "heat sphere". The

temperature is very high in this layer because

ultraviolet radiation is turned into heat.

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Temperatures often reach 2000°C or more. This

increase in temperature is due to the absorption of

intense solar radiation by the limited amount of

remaining molecular oxygen.

3. The atmosphere in the ionosphere is partly ionised

enriched ion zones exist in the form of distinct

ionised layers. So, this layer is also called as the

ionosphere. Long distance radio communication is

possible due to this layer.

E). Exosphere

1. The outer most layer of the earth’s atmosphere is

named as the exosphere and this layer lies between

400 and 1000 km. The exosphere is the most distant

atmospheric region from Earth's surface.

2. The upper boundary can be defined theoretically by

the altitude (about 120,000 miles, half the distance to

the Moon) at which the influence of solar radiation

pressure on atomic hydrogen velocities exceeds that of

the Earth's gravitational pull.

3. At such a greater height the density of atoms in the

atmosphere is extremely low.

4. Hydrogen and Helium gases predominate in this

outer most region.

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5. At an altitude of about 500 to 600 km the density of

the atmosphere becomes so low that collisions

between the natural particles become extremely rare.

Weather

It is defined as a physical state of the atmosphere

at a given place and given time or a state or condition

of the atmosphere at a given place and at a

The daily or short term variations of different

conditions of lower air in terms of temperature,

pressure, wind, rainfall etc. Weather is highly variable

and it changes constantly sometimes from hour to hour

and at other times from day to day.

The different weather elements are solar

radiation, temperature, pressure, wind, humidity,

rainfall, evaporation etc.

Eg: The air temperature of Annamalai nagar on

20.01.2011 at 10.30 a.m. is 31º C.

Climate

It is defined as the generalized weather or

summation of weather conditions over a given region

during comparatively longer period. The sum of all

statistical information of weather in a particular area

during specified interval of time, usually a season or a

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year or even a decade or long term regime of

atmospheric variables of a given place or area. The

aspects involved are larger areas like a zone/ a state/ a

country and is described as normals. The climatic

normals are generally worked out for a period of 30

years. The climatic elements are latitude, longitude

and altitude.

Eg: In Tamil Nadu the summer temperature ranges

from 29ºC to 40º C.

Factors affecting climate

There are number of factors that affect the

climate of a particular place in the World. They are,

i) Latitude

Latitude controls the amount of solar radiation

that reaches the surface of the earth. The distance from

the equator, either south or north, largely creates

variations in the climate. Based on the latitude, the

climate has been classified as

i) Tropical ii) Sub-tropical iii)

Temperate iv) Polar

The tropical climate is characterized by high

temperature throughout the year. Subtropical is

characterized by high temperature alternating with low

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temperature in winter. The temperate climate has low

temperature throughout the year. The polar climate is

noted for its very low temperature throughout the year.

In general the latitude incease the temperature

decrease.

ii) Altitude (elevation)

The height from the MSL (Mean Sea Level)

creates variation in climate. Even in the tropical

regions, the high mountains have temperate climate.

The temperature decreases by 6.5 ºC/km from the sea

level. Generally, there is also a decrease in pressure

and increase in precipitation and wind velocity. The

above factors alter the kind of vegetation, soil types

and the crop production. With increasing altitude the

decreasing temperature.

iii) Precipitation

The quantity and distribution of rainfall decides

the nature of vegetation and the nature of the

cultivated crops.

iv) Mountain barriers or Relief:

Mountains form a natural barrier that causes air

masses to rise. As air is forced to rise it expands as

gravity decreases, it becomes less dense and cools.

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v) Soil type

Soil is a product of climatic action on rocks as

modified by landscape and vegetation over a long

period of time. The colour of the soil surface affects the

absorption, storage and re-radiation of heat. White

colour reflects while black absorbs more radiation.

Hence, where ever black soil type is predominant there

will be more absorption of heat and hot climate will

exist.

vi) Nearness to large water bodies

The presence of large water bodies like lakes and

sea including its current affect the climate of the

surrounding areas, Eg. Islands and coastal areas. The

movement of air from earth’s surface and from water

bodies to earth modifies the climate. The extreme

variation in temperature during summer and winter is

minimized in coastal areas and island.

vii) Topography

The surface of landscape (leveled or uneven

surface areas) produces marked change in the climate.

This involves the altitude of the place, steepness of the

slope and exposure of the slope to light and wind.

viii) Vegetation

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Kinds of vegetation characterize the nature of

climate. Thick vegetation is found in tropical regions

where temperature and precipitation are high.

Others factors are

Wind and air masses, atmospheric disturbances

or storms and oceans currents.

Scales of climate and their importance

i) Microclimate

Microclimate deals with the climatic features

peculiar to small areas and with the physical processes

that take place in the layer of air very near to the

ground which differ sharply from general climatic

conditions.

ii) Mesoclimate

The scale of meso climate falls between micro

and macro climates. It is concerned with the study of

climate over relatively smaller areas between 10 & 100

km across.

iii) Macroclimate

Macro climate deals with the study of atmosphere over large

areas of the earth and with the large scale atmospheric motions that

cause weather. The scales of air motion in different climates are given

in the Table below.

Type of Horizontal Vertical Time 20

climate scale (km) scale(km) Scale

(hrs)

A. Macro

climate

1.

Planetary

scale

2000-5000&

more

10 200 to

400

2. Synoptic

scale

500-2000 10 100

B. Meso

climate

1 to 100 1-10 1-10

C. Micro

climate

<100m 200 m 6-12

minutes

If any weather system develops under different types of

climate, it persists longer periods under the

macroclimate while smaller periods under micro

climates.

Climates of India and Tamilnadu and their

characterization

Climate classification was tried by many

scientists from beginning of 19th century using many

parameters. Thornthwaite during 1931 and 1948

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classified the climate using precipitation and

evaporation /Potential evaporation and was

subsequently modified by Mathur (1955) for the

Moisture Index (Im) and is given below

Im = 100 [(P-PE)/PE]

Where P = Precipitation, PE = Potential

evapo-transpiration

Using the moisture Index (Im) the following

classification was made

Im Quantity Climate classification

100 and above Per humid

20 to 100 Humid

0 to 20 Moist sub humid

-33.3 to 0 Dry sub humid

-66.7 to -33.3 Semi arid

-100 to -66.7 Arid

Another classification by Troll (1965) based on

number of humid months, said to be of more

agricultural use was modified by ICRISAT for India.

Humid month is one having mean rainfall exceeding the

mean Potential evapo transpiration.

Climate Number of humid months

% geographical area of India

Arid <2.0 17.00

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Semiarid-dry

2.0-4.5 57.17

Semiarid-wet

4.5-7.0 12.31

Humid >7.0 1.10The ICAR under All India Coordinated Research Project

on Dryland Agriculture adopted classification based

Moisture Deficit Index (MDI)

P - PETMDI = --------- x 100

PETWhere P is annual precipitation (cm) and PET is Potential

Evapotranspiration. Based on MDI the climate is divided into three

regions as below.

Type of climate MDI

Subhumid 0.0 to 33.3

Semiarid -33.3 to -66.6

Arid > -66.6

Koppen classification of climate

Koppen classified climate into eleven types. They

are as follows:

Sl.No.

Name Sl.No.

Name

1. Tropical rainforest climate

7. Humid temperate climate

2. Tropical –Savanna Climate

8. Cold climate with moist winter

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3. Steppes 9. Cold Climate with Dry Winter

4. Deserts 10. Tundra Climate5. Warm climate

with dry winter11. Ice climate

6. Warm Climate with dry summer

Rainfall based

classification

The crop regions are

classified on the basis of

average rainfall which is as follow.

Rainfall(mm) Name of the climatic

region

Less than 500 Arid

500-750 Semi-arid

750-1000 Sub-arid

More than 1000 Humid

Temperature based classification

The tropic of cancer, which passes through the

middle of the country, divides it into two distinct

climates. The tropical climate in the South where all the

12 months of the year have mean daily temperature 24

exceeding 20°C; and in the North where a sub-tropical

climate prevails. In sub-tropics during the winter

months, it is cool to cold. Frosts occur sometime during

the months of December and January. Some areas in

the Northern India have a temperate climate. Here it

snows during the winter months and freezing

temperatures may extend to two months or more

during the year. Three main climatic zones of India

based on temperature are shown in the map below.

Chapter 2

Solar Radiation and Temperature

THE SUN

Sun is an ordinary star, nearest to our Earth. The

planets and satellites are called the members of the

solar system. Sun is the star at the center of the Solar

System. All the planets revolve around the Sun in

elliptical orbits. Sun is the prime source of energy on

Earth and life depends on.

The main features of the Sun

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1. Diameter of the Sun is 13,92,000 km and its mass

about 2 X 1030 kg

(3,30,000 times that of Earth)

2. It rotates on its axis about once in every four weeks

(27 days near equator & 30 days –polar)

3. The mean distance of the Sun from the Earth is

approximately 149.6 million km

4. Sun is the brightest object in the sky

5. Light travels from the Sun to Earth in about 8

minutes and 19 seconds

6. The surface temperature of the Sun is around 5778°K

(5505 °C)

7. The interior mass of the Sun has a density of 80 to

100 times that of water

8. Every minute, the Sun radiates approximately 56 X

1026 calories of energy

9. Energy is due to the fusion, Hydrogen is transformed

to Helium

10. 99% of the energy to biosphere is only from the Sun

and the rest one

per cent is from stars, lightning discharge. Sun’s

radiation reflected from the Moon, re-radiation from the

Earth etc.,

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SEASONAL VARIATIONS

The primary cause of seasonal extremes on Earth

is the 23.5 degree tilt of our planet's spin axis. When

the north pole is tilted toward the Sun, northern days

are long and the weather is warm. Six months later, as

the south pole tilts toward the Sun, the southern

hemisphere takes its turn at summer.

Seasons in the two hemispheres are always

reversed. When it is summer in New York, it is winter in

Sydney. On a spring day in Paris, autumn leaves are

falling in Argentina.

The name "equinox" is derived from the Latin

aequus (equal) and nox (night). The name “solstice” is

derived from the Latin sol (sun) and sistere (to stand

still)

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EQUINOX - when the Sun shines down directly

over a planet's equator. Two times of the year when

night and day are about the same length.

SOLSTICE - when one of the poles is tilted

toward the Sun to its maximum extent. The Sun stands

still in declination; that is, the apparent movement of

the Sun's path north or south comes to a stop before

reversing direction.

Summer Solstice: The Sun is farthest north and

the length of time between Sunrise and Sunset is the

longest of the year.

Winter Solstice: The Sun is farthest south and

the length of time between Sunrise and Sunset is the

shortest of the year.

An equinox happens each year at two specific

moments in time, where the center of the Sun can be

observed to be vertically overhead, occurring around

March 21 and September 23 each year.

A solstice is an astronomical event that happens

twice each year when the Sun's apparent position in

the sky reaches its northernmost or southernmost

extremes. From the vernal equinox (March 21), the

Earth is moving northward around the Sun towards the

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summer solstice (June 21), when the Sun is farthest

North. The autumnal equinox (September 23), the Earth

moving southward towards winter solstice (December

21), the Sun reaches its lowest position in the sky

(South).

If you live in the Southern Hemisphere, the

summer solstice and winter solstice will be reversed.

INSOLATION

Insolation means Incoming solar radiation.

Electromagnetic energy radiated into the space by the

Sun. It consists of bands of radiant energy of different

wave lengths.

Factors affecting insolation

1. The solar constant which depends on

a. Energy output of the Sun

b. Distance from earth to Sun

2. Transparency of the atmosphere

3. Duration of daily sunlight period

4. Angle at which Sun’s noon rays strike the Earth.

SOLAR CONSTANT

Solar constant is the energy received on a unit

area at the outer most boundary of the Earth

(atmosphere) surface held perpendicular to the Sun’s

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direction, at the mean distance between the Sun and

the Earth.

The rate at which solar wave radiation is received

at the top of the atmosphere on a unit surface unit

time.

Solar constant is not a true constant. It fluctuates

by as much as ± 3% about its mean value

depending upon the distance of the Earth from

the Sun.

Value is about 2 cal / cm2 / min. (1.92 and 2.02) of

the upper atmosphere (cal/cm2= Langley). It is

equal to 1370 watts per square meter

35% of the energy is contributed by U.V. and

visible parts and 65% by Infra Red

TRANSFER OF HEAT

All matter, at a temperature above the absolute

zero, imparts energy to the surrounding space. Three

processes viz. conduction, convection and radiation are

involved in heat flow or heat transfer.

1. Conduction

Heat transfer through the matter without the

actual movement of molecules of the substances or

matter. Heat flows from the warmer to cooler part of

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the body so that the temperatures between them are

equalized.

Eg. The energy transmission through an iron rod which

is made warmer at one end.

2. Convection

Processes of transmission of heat through actual

movement of molecules of the medium. This is

predominant form of energy transmission on the earth

as all the weather related processes involve this

process.

Eg. Boiling of water in a vessel.

3. Radiation

Transfer of energy from one body to another

without the aid of the material medium (solid, liquid or

gas). Radiation is not a heat, only when radiation is

absorbed by surface of a body heat is produced.

Eg. The energy transmission through space from the

Sun to the Earth.

SOLAR RADIATION

The flux of radiant energy from the Sun is solar

radiation.

Heavenly bodies emit – short wave radiation

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Near surfaces including earth emit - long wave

radiation

Importance of Solar Radiation on Crops

1. From germination of seeds to harvesting and even

post harvest processes are affected by solar radiation

2. Solar radiation provides energy for

All the phenomena related to biomass production

All photosynthetic processes

All physical processes taking place in the soil, the

plant and their environment

3. Solar radiation controls the distribution of

temperature thereby distribution of crops into different

regions.

Radiation flux

The amount of radiant energy emitted, received,

transmitted across a particular area is known as radiant

flux.

Radiant flux density

The radiant flux divided by the area across which

the radiation is transmitted is called radiant flux

density.

Emissive power

32

The radiant flux density emitted by a source is

called its emissive power.

Energy measurement

Units Cal cm-2

min-1

J cm-2 mi-1 W cm-2

Cal cm-2

min-1

1 4.1868 0.069

J cm-2 mi-1 0.238 1 0.00165

W cm-2 14.3 60.6 1

Where,

J – Joule; W – Watt.

SOLAR SPECTRUM

Radiation energy is transmitted in the form of

electromagnetic waves by the Sun. The energy from

the Sun is spread over a very broad band of

wavelengths known as solar spectrum. It is also known

as electromagnetic spectrum. The spectrum is the

combination of different wavelengths.

Eg. U.V. rays, visible light, I.R.rays etc.

Different Bands in Solar Spectrum

BandSpectr

um

Wavelength(µ)

Importance

% of

Energy

33

1.Ultra violet

Cosmic rays

< 0.005Shorter wave lengths of spectrum & Chemically active, unless filtered there is danger of life on earth

9%

Gamma rays and X-rays

0.005 – 0.20

Ultra Violet rays

0.20 – 0.39

2.Visible

Violet0.39 – 0.42

Visible spectrum known as Light essential for all plant processes

41%

Blue0.42 – 0.49

Green0.49 – 0.54

Yellow0.54 – 0.59

Orange0.59 – 0.65

Red0.65 – 0.76

3.Infra red

Infrared rays

> 0.76

Essential for thermal energy of the plant (Source of heat)

50%

Units of measurements of wavelength

Micron (µ) = 10-6 m = 10-4 cm

Milli micron (mµ) = 10-9 m = 10-7 cm

Angstrom (Å) = 10-10 m = 10-8 cm

34

Solar radiation and crop plants

Crop production is exploitation of solar radiation;

the three broad spectrums are,

1. Shorter than visible range: Chemically very

active

When plants are exposed to this radiation the

effects are detrimental

Atmosphere acts as regulator for this radiation

and none of cosmic, Gamma and X-rays reaches

to the earth

The UV rays of this segment reaching to the earth

are very low and it is normally tolerated by the

plants

2. Visible spectrum: Light

Between UV & IR radiation and also referred as

light

All plant parts are directly or indirectly influenced

by the light

Intensity, quality and duration are important for

normal plant growth

Poor light leads to plant abnormalities

Light is indispensable to photosynthesis

35

Light affect the production of tillers, the stability,

strength and length of culms

It affects the yield, total weight of plant

structures, size of the leaves and root

development

It affects the flowering and fruiting and dormancy

of the seed

3. Higher than visible wavelength: Source of heat

Referred to IR radiation

It has thermal effect on plants

In the presence of water vapour, this radiation

does not harm plants, rather it supplies the

necessary thermal energy to the plant

environment

Effect of solar radiation on crop plants

S.No.

Band Wavelength (µ)

Specific effect on plant

1.Radiation within 1.0 and more

No specific effect on plant activity such as bio-chemical and photochemical processes. Radiations are transformed into heat

2. 1.0 - 0.72Radiation in this band helps in plant elongation activity

3. 0.72 – 0.61

This light is strongly absorbed by chlorophylls. It generates

36

strong photosynthetic and photo-periodic activity

4.0.61 – 0.51

Absorption in this region has low photosynthetic effectiveness and weak formative activity

5.0.51 – 0.40

It is the strongest chlorophyll and yellow pigment absorption region. Very strong on photosynthetic activity and effect on formation of tissues

6.0.40 – 0.32

Produces formative effects. It has dwarfing effect on plants and thickening effect on plant leaf

7.0.32 – 0.28

Detrimental effect on most plants

8.Less than 0.28

Lethal effects most of the plants get killed UV ranges have germicidal action

Absorption, Reflection and Transmission

A part of the incident radiation on the surface is

absorbed, while a part is reflected and the remaining is

transmitted.

Absorptivity

Absorptivity of a substance is defined as the ratio

of the amount of radiant energy absorbed to the total

amount incident upon that substance. The absorptivity

of a blackbody is unity. Natural bodies like Sun and

Earth are near perfect black bodies.

37

Reflectivity

Reflectivity is defined as the ratio of the radiant

energy reflected to the total incident radiation upon

that surface. If it is expressed in percentage it becomes

albedo.

Transmittivity

Transmittivity is defined as ratio of the

transmitted radiation to the total incident radiation

upon the surface.

Emissivity

Emissivity is defined as the ratio of the radiant

energy emitted by a given surface to the total heat

energy emitted by a black body. The emissivity of a

black body is unity.

Blackbody radiation

A Blackbody is defined as a body, which

completely absorbs all the heat radiations falling on it

without reflecting and transmitting any of it. It means

reflectivity and transmittivity become zero. When such

a black body is heated it emits radiation of all

wavelengths depending upon its temperature.

ALBEDO

38

It is the percentage of reflected radiation to the

incident radiation (varies with colour and composition

of the Earth’s surface, season, angle of the Sun rays).

Value is Highest in winter and at Sunrise and Sunset.

Pure water 5-20%, Vegetation 10-40%, dark soils 5-

10%, desert soil 25-30%, snow 45-90% and clouds 5-

85%. High albedo indicates that much of the incident

solar radiation is reflected rather than absorbed.

Depends up on

1. Angle of incidence of radiation. Albedo increase

with decreasing elevation of sun with minimum

during noon

2. Physical characteristics of surface

3. Season

4. Time of the day

For plant community albedo depends upon

1. Age of the crop

2. Percentage of ground cover

3. Colour and reflectivity of the foliage

Radiation balance

The difference between all incoming and

outgoing radiation at the earth’s surface and top of the

39

atmosphere is known as radiation balance at the

earth’s surface.

Temperature

It is defined as the measure of the average speed

of atoms and molecules

Kinetic energy

Energy of motion.

Heat

It is the aggregate internal energy of motion and

molecules of a body. It is often defined as energy in the

process of being transferred from one object to another

because of the temperature between them.

Sensible heat

It is the heat that can be measured by a

thermometer and thus sensed by humans. Normally

measured in Celsius, Fahrenheit and Kelvin.

Latent heat

It is the energy required to change a substance to

a higher state of matter. This same energy is released

on the reverse process. Change of state through

Evaporation and condensation is known as latent heat

of evaporation and latent heat of condensation. From

40

water to water vapour takes 600 calories and water to

ice takes 80 calories.

Disposition of Solar radiation

25% of solar radiation is reflected back to the

space by clouds (more by middle and high

latitudes and less in the sub tropics)

6% reflected back by air, dust and water vapour.

30% scatted downwards (more in the form of

shorter wavelengths than that in longer wave

length (red)).

17% of solar radiation is absorbed by the

atmosphere. (Mostly by Oxygen, O3, CO2 & H2O

vapour).

O2 – absorb the extreme UV wavelengths

(0.12 to 0.6 µ)

O3 – UV (0.2 to 0.32 µ) and Visible part of

radiation (0.44 to 0.7 µ)

H2O vapour – Near infra red (0.93, 1.13, 1.42 µ)

CO2 - IR band 2.7 µ.

About 50% of solar radiation reaches earth’s

surface, after reflection, scattering and absorption.

LIGHT

41

Light is the visible portion of the solar spectrum

with wavelength range is from 0.39 to 0.76μ. Light is

one of the important climatic factors for many vital

functions of the plant.

Functions of Light

It is essential for the synthesis of the most

important pigment ie. Chlorophyll, Chlorophyll

absorbs the radiant energy and converts it into

potential energy of carbohydrate

(photosynthesis).

The carbohydrate thus formed is the connecting

link between solar energy and living world.

In addition, it regulates the important

physiological functions.

- Effects tiller production, stability, strength and

length of culms

- Effects dry matter production

- Effects the size of the leaves and root

development

- Effects the flowering, fruiting and dormancy of

the seed

Characteristics of Light

42

The characteristics of light viz. intensity, quality,

duration and direction are important for crops.

1. Light intensity

The intensity of light is measured by comparing

with a standard candle. The amount of light received at

a distance of one meter from a standard candle is

known as “Meter candle or Lux”. The light intensity at

one foot from a standard candle is called “foot candle”

or 10.764 luxes.

- Very low light intensity reduces the rate of

photosynthesis resulting in reduced growth.

- Similarly, very high intensity is detrimental to

plant in many ways as below.

- It increases the rate of respiration.

- It also causes rapid loss of water (i.e.) increases

the transpiration rate of water from the plants.

- The most harmful effect of high intensity light is

that it oxidises the cell contents which is termed

as ‘Solarisation’. This oxidation is different from

respiration and is called as photo-oxidation.

Under field conditions the light is not spread evenly

over the crop canopy but commonly passed by

43

reflection and transmission through several layers of

leaves.

The intensity of light falls at exponential rate with

path length through absorbing layers according to

Beer’s law. i.e. the relative radiation intensity

decreases exponentially with increasing leaf area.

At ground level the light intensity is below the light

compensation point (The light intensity at which the

gas exchange resulting from photosynthesis is equal

to that resulting from respiration)

Based on the response to light intensities the

plants are classified as follows.

(i) Sciophytes (shade loving plants)

The plants grow better under partially shaded

conditions. Eg. Betel vine, buck wheat etc.

(ii) Hetrophytes (Sun loving)

Many species of plants produce maximum dry

matter under high light intensities when the

moisture is available at the optimum level.

Eg. Maize, sorghum, rice etc.

2. Quality of Light

When a beam of white light is passed through a prism,

it is dispersed into wavelengths of different colours.

44

This is called the visible part of the solar spectrum

(VIBGYOR 0.39 – 0.76 µ).

The principal wavelength absorbed and used in

photosynthesis are in the violet – blue and the

orange - red regions.

Among this, short rays beyond violet such as X

rays, gamma rays and larger rays beyond red such

as infrared, are detrimental to plant growth.

Red light is the most favourable light for growth

followed by violet – blue.

Red light helps mature apples to turn red,

induces germination and suppresses elongation of

stem.

Blue and green lights inhibit germination of

seeds.

Ultra – violet and shorter wave lengths kill

bacteria and many fungi (Germicidal effect).

3. Duration of light:

The duration of light has greater influence than the

intensity for canopy development and final yield.

It has a considerable importance in the selection of

crop varieties.

45

The response of plants to the relative length of the

day and night is known as “photoperiodism”.

The plants are classified based on the extent of

response to day length which is as follows.

(i) Long day plants

The plants which develop and produce normally

when the photoperiod is greater than the critical

minimum (more than 10 hours). Eg. Potato, Sugar beet,

Wheat, Barley, Oats etc.

(ii) Short day plants

The plants which develop normally when the

photoperiod is less than the critical maximum (less

than 10 hours). Eg. Rice, Sorghum, Cotton, Sunflower,

Cucumber etc.

(iii) Day neutral plants / Indeterminate

Those plants which are not affected by photoperiod.

Eg. Tomato, Maize etc.

The photoperiodism influences the plant character

such as floral initiation or development, bulb and

rhizome production etc.

In long day plant, during periods of short days, the

growth of internodes are shortened and flowering is

delayed till the long days come in the season.

46

Similarly when short day plants are subjected to

long day periods

The varying photoperiodicity of plants is a very

important factor in crop rotation which helps not

only in operational nutrient management but also in

disease eradication.

The choice of varieties and cultivars in newly

cultivated areas depend on duration of light.

4. Direction of light

The direction of sunlight has a greater effect on the

orientation of roots and leaves.

In temperate regions, the southern slopes plants

produce better growth than the northern slopes due

to higher contribution of sunlight in the southern

side.

The change of position or orientation of organs of

plants caused by light is usually called as

“phototropism” i.e. the leaves are oriented at right

angles to incidence of light to receive maximum

light radiation.

Photomorphogenesis

Change in the morphology of plants due to light.

This is mainly due to U.V and violet ray of the Sun.

47

Factors Affecting the Distribution of Solar

Radiation within the Plant Canopy

1. Type of Plants

a. The leaves of cereal crops like paddy, wheat etc.

have a transmissivity from 5 to 10 per cent.

b. The broad leaves of ever green plants have lower

value of 2 to 8 per cent.

2. Age of the Leaves

The transmissivity of younger leaves is more as

compared to older leaves.

3. Chlorophyll Content

As the chlorophyll content increases the value of

transmissivity decreases.

4. Arrangement of Leaves

a. The relative light interception by horizontal and erect

foliage is 1:0.44

b. When the leaf area index is one (1) the light

transmissivity of more upright leaves is 74 % as against

50 % for horizontal leaves.

5. Angle of Leaves

a. In full sunlight, the optimum inclination for efficient

light use is 81°. The leaf placed at the optimum

48

inclination is 4 – 5 times as efficient in using light as a

horizontal leaf.

c. The ideal arrangement of leaves is that the lowest

leaves lay at angles between 0° to 30° to be horizontal

(13 % ), the adjoining leaves lay at 30° to 60° (37 % )

and the upper leaves lay at 60° to 90°(50 % ).

6. Plant Density

In sparse crop stands not only the per cent of

light transmissivity is more but it is also variable with

the time of the day. In dense crop canopies the light

transmissivity is less.

7. Plant Height

When the plant height increases the

transmissivity of light by the canopy decreases.

8. Angle of the sun

a. The highest radiation penetration also occurs at

noon.

b. Relatively high radiation penetration also occurs both

in the morning as well as before the sunset due to high

proportion of diffuse light.

Instruments to measure Solar Radiation

49

1. Pyranometer: Total short wave radiation or

components on a horizontal plain surface from

hemispherical sky.

2. Net pyranometer or solarimeter: Net short wave

radiation in W m-2.

3. Pyrradiometer: Global or total radiation from sky

on a horizontal surface in W m-2.

4. Net pyrradiometer or net radiometer: Net short

wave and long wave radiation in W m-2.

5. Pyrheliometer: Direct solar beam on a plane

surface at normal incidence in W m-2.

6. Pyrgeometer: Net infrared radiation of the

atmosphere in W m-2.

7. Luxmeter or Photometric sensors: The

brightness (intensity) is measured in foot candles or

lux.

8. Photometer or light meter: Measured the light.

9. Sunshine shine recorder: The number of hours of

bright sunshine is recorded.

AIR TEMPERATURE

Temperature is defined as, “The measure of

speed per molecule of all the molecules of a body”.

Whereas heat is, “the energy arising from random

50

motion of all the molecules of a body” (Degree of

molecular activity). It is the intensity aspect of heat

energy.

Temperature is a measure of “the average

energy of the molecules of a body”, whereas heat is a

measure of “the total amount of thermal energy in a

body”. For example, the temperature of a cup of boiling

water is the same as that of a large pot of boiling water

(212°F, or 100°C), the large pot has more heat, or

thermal energy, and it takes more energy to boil a pot

of water than a cup of water.

In meteorology, the temperature of the

atmosphere which represents the average kinetic

energy of the molecular motion in a small region and is

defined in terms of a standard or calibrated

thermometer in thermal equilibrium with the air.

The degree of hotness or coldness of a body or

environment.

HEAT TRANSFER

Conduction

Heat transfer when two bodies of unequal

temperatures come into contact. Heat passes from

point to point by means of adjacent molecules.

51

Convection

Transfer through movement of particles (part of

mass) in fluids and gasses. These are able to circulate

internally and distribute heated part of the mass.

Radiation

It is the process of transmission of energy by

electromagnetic waves between two bodies without the

necessary aid of an intervening material medium.

Role of temperature in crop production:

1. Temperature influences distribution of crop

plants and vegetation.

2. The surface air temperature is one of the

important variables, which influences all stages of

crop during its growth development and

reproductive phase.

3. Air temperature affects leaf production,

expansion and flowering.

4. The diffusion rates of gases and liquid changes

with temperature.

5. Solubility of different substances is dependent

on temperature.

52

6. Biochemical reactions in crops (double or more

with each 10°C rise) are influenced by air

temperature.

7. Equilibrium of various systems and compounds

is a function of temperature.

8. Temperature affects the stability of enzymatic

systems in the plants.

9. Most of the higher plants grow between 0°C –

60°C and crop plants are further restricted from 10

– 40°C, however, maximum dry matter is produced

between 20 and 30°C

10. At high temperature and high humidity, most of

the crop plants are affected by pests and diseases.

11. High night temperature increases respiration

and metabolism.

12. Most of the crops have upper and lower limits

of temperature below or above which, they may not

come up and an optimum temperature when the

crop growth is maximum. These are known as

“cardinal temperatures” and different crops have

different temperatures.

Sl No

Crop Minimum Optimum Maximum

1 Wheat and 0 – 5 25 – 31 31 – 37

53

Barley2 Sorghum 15 – 18 31 – 36 40 – 42

Factors affecting air temperature

Latitude

Altitude

Distribution of land and water

Ocean currents

Prevailing winds

Cloudiness

Mountain barriers

Nature of surface

Relief

Convection and turbulence etc.

1. Latitude

The time of occurrence of maximum monthly

mean temperature and minimum monthly mean

temperature also depends on latitude of a place. (Eg.)

The coldest month is January in northern regions of

India while December in the south. Similarly, the

warmest month is May in the south while June in the

north across the country.

2. Altitude

54

The surface air temperature decreases with

increasing altitude from the mean sea level as the

density of air decreases. Since the density of air is less

at higher altitudes, the absorbing capacity of air is

relatively less with reference to earth’s long wave

radiation.

3. Distribution of land and water

Land and water surfaces react differently to the

insolation. Because of the great contrasts between land

and water surfaces their capacity for heating the

atmosphere varies. The differential heating process

between land and sea surfaces are due to their

properties. It is one of the reasons for Indian monsoon.

4. Ocean currents

The energy received over the ocean surface

carried away by the ocean currents from the warm

areas to cool areas. This results in temperature contrast

between the equator and poles. The occurrence of El-

Nino is due to change in sea surface temperature

between two oceanic regions over the globe.

5. Prevailing winds

Winds can moderate the surface temperature of

the continents and oceans. In the absence of winds, we

55

feel warm in hot climates. At the same time, the

weather is pleasant if wind blows.

6. Cloudiness

The amount of cloudiness affects the

temperature of the earth’s surface and the atmosphere.

A thick cloud reduces the amount of insolation received

at a particular place and thus the day time temperature

is low. At the same time, the lower layers in the

atmosphere absorb earth’s radiation. This results in

increasing atmospheric temperature during night. That

is why, cloudy nights are warmer. This is common in

the humid tropical climates.

7. Mountain barriers

Air at the top of the mountain makes little

contact with the ground and is therefore cold while in

the valley at the foothills makes a great deal of contact

and is therefore warm. That is, the lower region of the

earth’s atmosphere is relatively warmer when

compared to hillocks.

8. Nature of surface

9. Relief

56

In the northern hemisphere north facing slopes

generally receive less insolation than south facing

slopes and temperature are normally lower.

10. Convection and turbulence etc.

Diurnal and Seasonal Variation of Air

Temperature

The minimum air temperature occurs at about

sunrise, after which there is a constant rise till it

reaches to maximum.

The maximum air temperature is recorded between

13.00 hrs and 15.00 hrs although the maximum

solar radiation is reaches at the noon.

A steady fall in temperature till sunrise is noticed

after is attains maximum. Thus the daily displays

one maximum and one minimum. The difference

between the two is called the diurnal range of air

temperature.

The diurnal range of air temperature is more on

clear days while cloudy weather sharply reduces

daily amplitudes.

The diurnal range of temperature is also influenced

by soils and their coverage in addition to seasons.

57

Addition of daily maximum and minimum

temperature divided by two is nothing but daily

mean / average temperature.

Temperature Distribution

Each day the earth receives energy in the form of

incoming solar radiation from the sun. There are three

temperature distributions viz., horizontal, vertical and

periodic distributions that are observed on earth.

1. Horizontal air temperature distribution

Sun rays make different angles at the same place

at different times. Due to the variation in angle of sun’s

rays, distribution of solar heat on earth decreases both

ways from equator to poles. This is known as horizontal

distribution of air temperature.

On maps, the horizontal distribution of air

temperature is shown by isotherms. The isotherms are

imaginary lines drawn connecting points of equal

temperature.

Horizontal distribution of air temperature influenced

by,

Latitude

58

A general decrease in temperature from equator

towards poles is one of the most fundamental factors of

climatology.

Land and water

Irregular distribution of land and water on earth’s

surface breaks the latitudinal variation in temperature.

Land areas warm and cool rapidly than water bodies.

Mountain barriers

Influence horizontal distribution of temperature by

restricting movement of air masses. Eg. Himalayas

protect India from polar air.

Topography

On local scale topographic relief exerts an

influence on temperature distribution. In northern

hemisphere north facing slopes receive less insolation

than south facing slopes and temperatures are

normally lower.

2. Vertical air temperature distribution

The decrease in temperature with increase in

height (altitude) is known as vertical temperature

distribution. Eg. Permanent snow caps in high

mountains.

3. Periodical temperature distribution

59

The air temperature changes continuously during a day

or a year.

Maximum insolation is received around noon

(12.00 hr) at maximum temperature is recorded

from 1 p.m. to 3 p.m. and delay is known as

thermal lag.

In northern hemisphere winter minimum occurs in

January and summer maximum in July.

Temperature and Climatic Zones

The climatic zones of the earth are:

Tropical

Sub – tropical

Temperate

Alpine

These zones are differentiated on the basis of

temperature. The tropical zone is a hot winterless zone;

sub-tropical zone is a hot zone with a cool winter;

temperate zone is with a warm summer and

pronounced winter; and the alpine zone has a short

summer and a long severe winter (Himalayas, Tibetan

Plateau).

Temperature inversion

60

Occasionally at some altitude the temperature

abruptly increases instead of decreasing. This condition

in which this “abrupt rise instead of fall in temperature

occurs in the air” is known as the temperature

inversion. This may occurs under the following

conditions.

When the air near the ground cools off faster than

the overlying layer, because of heat loss during

cooling nights.

When an actual warm layer passing over a lower

cold layer

Cold air from hill tops and slopes tend to flow

downward and replaced by warm air

Significance of Temperature inversion

Cloud formation, precipitation and atmospheric

visibility are greatly influenced by inversion

phenomenon

Impurities like smoke, dust, etc., are confined to

lower layers

Fog formation may take place near the ground which

may affect the visibility to both human beings and

animals. Affects air navigation.

61

Diurnal temperature (day and night) is affected by

temperature inversions.

The incoming solar radiation and its conversion into

heat is affected.

Heat Units

It is a measure of relative warmth of growing

season of a given length. Normally it is indicated as

“Growing Degree Days” (GDD) or “thermal units” or

“effective heat units” or “growth units”. A heat unit is

the departure from the mean daily temperature above

the minimum threshold temperature.

The growth of plants is dependent on the total amount

of heat to which it is subjected during its life time.

The minimum threshold temperature is the

temperature below which no growth takes place.

Usually ranges from 4.5 to 12.5 ºC for different crops

(tropical crop 10ºC).

Degree Day Concept

A degree day is obtained by subtracting the

threshold temperature from daily mean temperature.

Summation of the daily values over the growth period

gives degree days of the crops.

Tmax + Tmin

62

GDD = Σ ------------------- - Tb

2

Where

Tmax – Maximum air temperature of the day

Tmin – Minimum air temperature of the day

Tb - Base temperature of the crop

The base temperature is the threshold temperature.

Advantages / Importance of growing degree Day

Concept

In guiding the agricultural operations and planting

land use.

To forecast crop harvest dates, yield and quality

In forecasting labour required for agricultural

operations

Introduction of new crops and new varieties in new

areas

In predicting the likelihood of successful growth of a

crop in an area.

Air Temperature and Plant Injury

The injury to crop plants by air temperature is by

both low and high air temperatures.

Thermal death point

63

The temperature at which the plant cell gets

killed when the temperature ranges from 50-60°C. This

varies with plant species. The aquatic and shade loving

plants are killed at comparatively lower temperature

(40°C).

I. High Temperature Injury

- results in desiccation of plants

- disturbs the physiological activities like

photosynthesis and respiration

- increases respiration leading to rapid

depletion of reserve food.

1. Sun clad

Injury caused on the barks of stem by high

temperature during day time and low temperature

during the night time.

2. Stem griddle

The stem at ground level scorches around due to

high soil temperature. It causes death of plant by

destroying conductive tissues. Eg. This type of injury is

very common in young seedlings of cotton in sandy soil

when soil temperature exceeds 60°C.

64

II. Low Temperature (COLD) Injury

The primary effect of low air temperature below

that optimum temperature is the reduction of rates of

growth and metabolic processes.

(i) Suffocation

In temperature regions, usually during the

winter season, the ice or snow forms a thick cover on

the soil surface. As a result, the entry of oxygen is

prevented (deficient oxygen) and crop suffers for want

of oxygen. Ice coming in contact with the root prevents

the diffusion of CO2 outside the root zone. This prevents

the respiratory activities of roots leading to

accumulation of harmful substances.

(ii) Heaving

This is a kind of injury caused by lifting up of the

plants along with soil from its normal position. This type

of injury is commonly seen in temperate regions. The

presence of ice crystals increases the volume of soil.

This causes mechanical lifting of the soil.

(iii) Chilling injury

Plants which are adapted to hot climate, if exposed

to low temperature (0 to 10°C) for sometime, are found

to be killed or severely injured or development of

65

chloratic condition (yellowing) (Eg.) chloratic bands on

the leaves of sugarcane, sorghum and maize in winter

months when the night temperature is below 20°C.

(iv) Freezing injury

This type of injury is commonly observed in

plants of temperate regions. When the plants are

exposed to very low temperature, water freezes into ice

crystals in the intercellular spaces of plants. The

protoplasm of cell is dehydrated resulting in the death

of cells.

(Eg.) Frost damage in potato, tea etc.

Thermo periodic response

Response of living organism to regular changes

in temperature either day or night or seasonal is called

thermoperiodism.

Instruments to measure the air temperature

1. Maximum Thermometer: To record the maximum

temperature in a day.

2. Minimum Thermometer: To record the minimum

temperature in a day

3. Dry bulb Thermometer: To record the temperature at

a time.

66

4. Wet bulb Thermometer: To record the temperature

at a time under wet condition.

5. Thermograph: To record the temperature in a day

(continuous recording on a chart).

SOIL TEMPERATURE

Soil thermal properties are strongly influenced by

the soil volumetric water content, volume fraction of

solids and volume fraction of air. Air is a poor thermal

conductor and reduces the effectiveness of the solid

and liquid phases to conduct heat. While the solid

phase has the highest conductivity it is the variability of

soil moisture that largely determines thermal

conductivity.

Temperature variations are most extreme at the

surface of the soil and these variations are transferred

to sub surface layers but at reduced rates as depth

increases. Additionally there is a time delay as to when

maximum and minimum temperatures are achieved at

increasing soil depth (sometimes referred to as thermal

lag).

The temperature of the soil is an indicator of the

energy needed to sustain the normal activity of plants

and soil organisms. The soil temperature is one of the

67

most important factors that influence the crop growth.

Temperature can have a profound impact on the

success of seed germination, root and shoot growth,

nutrient uptake, crop growth and micro organisms live

in the soil. The physio-chemical as well as life processes

are directly affected by the temperature of the soil.

Under the low soil temperature conditions nitrification

is inhibited and the intake of water by root is reduced.

In a similar way extreme soil temperatures injures plant

and its growth is affected.

Eg. On the sunny side, plants are likely to develop

faster near a wall that stores and radiates heat. If

shaded by the wall, however, the same variety may

mature later. In such cases soil temperature is an

important factor.

Importance of Soil Temperature on Crop Plants

The soil temperature influences many process.

1. Governs uptake of water, nutrients etc

needed for photosynthesis.

2. Controls soil microbial activities and the

optimum range is 18-30°C.

3. Influences the germination of seeds and

development of roots.

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4. Plays a vital role in mineralization of

organic forms of nitrogen.(increases with increases

in temperature)

5. Influences the presence of organic matter

in the soil (more under low soil temperature)

6. Affects the speed of reactions and

consequently weathering of minerals.

7. Influences the soil structure (types of clay

formed, the exchangeable ions present, etc.)

Factors Affecting Soil Temperature

Heat at ground surface is propagated downward

in the form of waves. The amplitude deceases with

depth. Both meteorological and soil factors contribute

in bringing about changes of soil temperature.

I) Meteorological Factors

1. Solar Radiation

a) The amount of solar radiation available

at any given location and point of time is directly

proportional to soil temperature.

b) Even though a part of total net radiation

available is utilised in evapotranspiration and

heating the air by radiation (latent and sensible

heat fluxes) a relatively substantial amount of solar

69

radiation is utilized in heating up of soil (ground

heat flux) depending up on the nature of surface.

c) Radiation from the sky contributes a

large amount of heat to the soil in areas where the

sun’s rays have to penetrate the earth’s

atmosphere very obliquely.

2. Wind

Air convection or wind is necessary to heat up

the soil by conduction from the atmosphere. (Eg.) The

mountain and valley winds influence the soil

temperature.

3. Evaporation and Condensation

a) The greater the rate of evaporation the more the

soil is cooled. This is the reason for coolness of

moist soil in windy conditions.

b) On the other hand whenever water vapour from

the atmosphere or from other soil depths condenses

in the soil it heats up noticeably. Freezing of water

generates heat.

4. Rainfall (Precipitation)

Depending on its temperature, precipitation can either

cool or warm the soil.

II. Soil Factors

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1. Aspect and Slope

a) In the middle and high latitudes of the northern

hemisphere, the southern slopes receive more

insolation per unit area than the northern exposure.

b) The south west slopes, are usually warmer than

the south east slopes. The reason is that the direct

beam of sunshine on the south east slope occur

shortly after prolonged cooling at night, but the

evaporation of dew in the morning also requires

energy.

2. Soil Texture

a) Because of lower heat capacity and poor thermal

conductivity, sandy soils warm up more rapidly than

clay soils. The energy received by it is concentrated

mainly in a thin layer resulting in extraordinary rise

in temperature.

b) Radiational cooling at night is greater in light

soils than in heavy soils. In the top layer, sand has

the greatest temperature range, followed by loam

and clay.

c) The decrease of range with depth is more rapid

in light soils than heavy soils when they are dry but

slower when they are wet.

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d) A soil with rough surface absorbs more solar

radiation than one with a smooth surface.

3. Tillage and Tilth

a) By loosening the top soil and creating a mulch,

tillage reduces the heat flow between the surface

and the sub soil.

b) Since, the soil mulch has a greater exposure

surface than the undisturbed soil and no capillary

connection with moist layers below, the cultivated

soil dries up quickly by evaporation, but the

moisture in the sub-soil underneath the dry mulch is

conserved.

c) In general soil warms up faster than air. The

diurnal temperature wave of the cultivated soil has

a much larger amplitude than that of the

uncultivated soil.

d) The air 2-3 cm above the tilled soil is often hotter

(10°C or above) than that over an untilled soil.

e) At night loosened ground is colder and more

liable to frost than the uncultivated soil.

4. Organic matter

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a) The addition of organic matter to a soil reduces

the heat capacity and thermal conductivity. But, the

water holding capacity increases.

b) The absorbtivity of the soil increases because of

the dark colour of the organic matter.

c) At night, the rapid flow of heat from sub-soil by

radiation is reduced with the addition of organic

matter because of its low thermal conductivity.

d) The darker the colour, the smaller the fraction of

reflected radiation.

e) The dark soils and moist soils reflect less than

the light coloured and dry soils.

5. Soil moisture

a) Moisture has an effect on heat capacity and heat

conductivity.

b) Moisture at the soil surface cools the soil through

evaporation.

c) Therefore, a moist soil will not heat up as much

as a dry one.

d) Moist soil is more uniform in temperature

throughout its depth as it is a better conductor of

heat than the dry soil.

Variations in soil temperature

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There are two types of soil temperature

variations; daily and seasonal variation of soil

temperature

1. Daily variations of soil temperature

a) These variations occur at the surface of the soil.

b) At 5 cm depth the change exceeds 10°C. At 20

cm the change is less and at 80 cm diurnal changes

are practically nil.

c) On cooler days the changes are smaller due to

increased heat capacity as the soils become wetter

on these days.

d) On a clear sunny day a bare soil surface is hotter

than the air temperature.

e) The time of the peak temperature of the soil

reaches earlier than the air temperature due to the

lag of the air temperature.

f) At around 20 cm in the soil the temperature in

the ground reaches peak after the surface reaches

its maximum due to more time the heat takes to

penetrate the soil. The rate of penetration of heat

wave within the soil takes around 3 hours to reach

10 cm depth.

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g) The cooling period of the daily cycle of the soil

surface temperature is almost double than the

warming period.

h) Undesirable daily temperature variations can be

minimised by scheduling irrigation.

2. Seasonal variations of soil temperature:

a) Seasonal variations occur much deeper into the

soil.

b) When the plant canopy is fully developed the

seasonal variations are smaller.

c) In winter, the depth to which the soil freezes

depends on the duration and severeness of the

winter.

d) In summer the soil temperature variations are

much more than winter in tropics and sub tropics.

75

Daily and monthly variation of soil temperature

76

Measurement of soil temperature

Soil thermometer: To measure the earth temperature

at shallow depth.

Units of Measurements

In Agro meteorology the two basic scales of

temperature used are:

a. Centigrade °C b. Fahrenheit °F

Relationship between °C and °F is

1.    °F = °C x 9/5 + 32

2.    °C = (F- 32) x 5/9

In meteorology for upper air temperature

measurements the absolute scale °A.

°A = 273 + °C.

77

Chapter - 3

ATMOSPERIC PRESSURE AND WIND SYSTEMS OF

THE WORLD

Pressure is defined as the force per unit area. The

atmospheric pressure is the weight of the air column

per unit area, above the earth surface extending to the

limits of the atmosphere. The earth’s atmosphere

weighs about 60,000 billion tones. At sea level, it exerts

a pressure of about 1 kg/cm2. It is expressed in

millibar (mb) (1000 millibar = 1 bar = 1 million dynes

per square centimeter). The average pressure over the

earth’s surface at sea level is 1013.25 millibar,

equivalent to the weight of a column of 76.0 cm (30

inches) (1cm of Hg =13.33mb) of mercury at 0º Celsius

or to a weight of air of 1033.3 gram per square cm.

Unequal heating of the earth and its atmosphere by the

sun and rotation of the earth bring about differences in

atmospheric pressure. Air pressure can tell us about

what kind of weather to expect as well. If a high

pressure system is on its way, often you can expect

cooler temperatures and clear skies. If a low pressure

system is coming, then look for warmer weather,

storms and rain.

78

Isobars (Greek word “isos” – equal and “baros” –

weight)

The distribution of pressure is represented on

maps by ‘isobars’. Isobars are defined as the imaginary

lines drawn on a map to join places having the same

atmospheric pressure.

Measuring Atmospheric Pressure

Any instrument that measures air pressure is

called a barometer. The first measurement of

atmospheric pressure began with a simple experiment

performed by Evangelista Torricelli in 1643.

The most common type barometer used are

aneroid barometer and mercury barometer.

Pressure systems of the world

The shape of the earth is not uniform and subjected

to uneven distribution of solar radiation, when it

revolves around the sun. The uneven distribution of

solar radiation over different regions of the globe leads

to contrast in surface air temperature. This results in

variations of surface atmospheric pressure systems,

which are known as standard atmospheric pressure

systems / belts. There are altogether seven alternating

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low and high pressure belts on the earth’s surface.

They are as follows:

Equatorial trough of low pressure (between 5°N

and 5°S)

Subtropical high pressure belt (Northern

hemisphere) (25° and 35°N)

Subtropical high pressure belt (Southern

hemisphere) (25° and 35°S)

Subpolar low pressure belt (Northern hemisphere)

(60° and 70°N)

Subpolar low pressure belt (Southern

hemisphere) (60° and 70°S)

Polar high (Northern hemisphere)

Polar high (Southern hemisphere)

The equatorial region receives more solar radiation

and thus the surface air temperature is high, which

creates lighter air near the ground compared to higher

latitudes. The above condition leads to low atmospheric

pressure over the equatorial region while sub-tropical

high pressure belts develop in both the hemispheres

between 25 and 35 degree latitudes due to relatively

low surface air temperature. It is due to low solar

radiation received due to inclined sun’s rays over the

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subtropical region when compared to the equatorial

belt. Likewise alternate low and high atmospheric

pressure belt systems are developed across the globe

from the equator to the poles.

70°

60°

35°

25°

0°

25°

35°

60°

70°

Variation in Atmospheric Pressure

(a)Variation with height (Vertical pressure

variation)

a. Pressure decreases with height.

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Equatorial low trough

Subpolar low

Subtropical high

Polar high

Polar high

Subtropical high

Subpolar low

b. This is natural since the height of the atmosphere

column above decreases with height.

(b) Horizontal pressure variation

a. Temperature and pressure are inverse to each

other. Along equator low pressure exists and in cold

polar latitudes polar high exists.

b. Horizontal variation of air pressure on the surface of

the earth are indicated by Isobars.

c. The horizontal variation of pressure depends on

Temperature, extent of water vapour, land water

realationship etc.,

(c )Diurnal pressure variation

a) There is a definite rhythm in the rise and fall of

the pressure in a day.

b) Radiational heating (air expansion) and

radiational cooling (air contraction) are the main

reasons for diurnal variation in the air pressure.

c) Diurnal variation is more prominent near the

equator than at the mid latitudes.

d) The areas closer to sea level record relatively

larger amount of variation than in land areas.

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e) Equatorial regions absorbs more heat than it

loses while the polar region gives up more heat

than they receive.

(d) Seasonal pressure variation

a) Due to the effect caused by annual variation in

the amount of insolation, distinct seasonal

pressure variations occur.

b) These variations are larger in the tropical region

than the mid latitude and polar regions.

c) Usually, high pressures are recorded over the

continents during the cold season and over the

oceans during the warm season.

Causes of variation

The atmospheric pressure changes continuously

due to several factors. The most important factors are

changes with temperature, altitude, water vapour

content and rotation of earth.

a) Temperature

Hot air expands and exerts low pressure. Cold air

contracts and exerts high pressure. So the equator has

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a low pressure due to prevalence of high temperature

but poles have a high pressure.

b) Altitude

At sea level, the air column exerts its full

pressure, but when we stand on a hill or when we go in

the upper layers of atmosphere, we leave a portion of

air which cannot exert its full pressure. At sea level, a

coastal town enjoys high pressure but on high altitude

one will register a low pressure. For every 10 m of

ascend, the pressure get reduced by 1 hPa.

c) Water vapour

The water vapour content is lighter in cold area

than in air which is dry with the result that moist air of

a high temperature exerts a less pressure when

compared to cold air.

d) Rotation of the earth

On account of rotation of the earth, the pressure

at 60-70°N and S becomes low. The rotation of the

earth near sub-polar belts, makes the air to escape

from these belts which move towards the horse latitude

(30° - 35°N and 30 – 35°S). These latitudes absorb the

air from sub polar belts making the pressure high.

G.D.Coriolis (1844) a French Mathematician indicated

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that air is deflected towards right in the Northern

Hemisphere and Left in the Southern hemisphere due

to rotation of earth and this was termed after him as

Coriolis force. Coriolis force is not actually a force but it

is effect created by rotation of earth.

Importance of pressure on crop plants

Pressure patterns

Low / Depression or cyclone

When the isobars are circular or elliptical in

shape, and the pressure is lowest at the centre, such a

pressure system is called ‘Low’ or ‘Depression’ or

‘Cyclone’. The movement will be anti-clockwise in the

Northern hemisphere while it is clockwise in the

southern hemisphere. Wind speed hardly exceeds 40

km per hour.

High pressure or Anticyclone

When isobars are circular, elliptical in shape and

the pressure is highest at the centre such a pressure

system is called ‘High’ or ‘Anticyclone’. When the

isobars are elliptical rather than circular the system is

called as ‘Ridge’ or ‘Wedge’. The movement will be

clockwise in the Northern hemisphere while it is anti-

clockwise in the southern hemisphere.

85

Atmospheric Humidity

The amount of water vapour that is present in

atmosphere is known as atmospheric moisture or

humidity.

Absolute humidity

The actual mass of water vapour present in a

given volume of moist air. It is expressed as grams of

water vapour per cubic meter or cubic feet.

Specific humidity

Weight of water vapour per unit weight of moist

air. It is expressed as grams of water vapour per

kilogram of air (g/kg).

Mixing ratio

The ratio of the mass of water vapour contained

in a sample of moist air to the mass of dry air. It is

expressed as gram of water vapour per kilogram dry

air.

Relative Humidity

The ratio between the amount of water vapour

present in a given volume of air and the amount of

water vapour required for saturation under fixed

temperature and pressure. There are no units and this

is expressed as percentage. In other terms it is the ratio

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of the air’s water vapour content to its maximum water

vapour capacity at a given temperature expressed in

percentage. The relative humidity gives only the

degree of saturation of air. The relative humidity of

saturated air is 100 per cent.

Dew Point temperature

The temperature to which a given parcel of air

must be cooled in order to become saturation at

constant pressure and water vapour content. In this

case, the invisible water vapour begins to condense

into visible form like water droplets.

Vapour Pressure deficit

The difference between the saturated vapour

pressure (SVP) and actual vapour pressure (AVP) at a

given temperature. This is an another measure of

moisture in the atmosphere which is useful in crop

growth studies. When air contains all the moisture that

it can hold to its maximum limit, it is called as

saturated air, otherwise it is unsaturated air, at that

temperature. The vapour pressure created at this

temperature under saturated conditions is vapour

pressure or saturated vapour pressure (SVP).

Importance of Humidity on crop plants

87

The humidity is not an independent factor. It is

closely related to rainfall, wind and temperature. It

plays a significant role in crop production

1. The humidity determines the crops grown in a

given region

2. It affects the internal water potential of plants

3. It influences certain physiological phenomena in

crop plants including transpiration

4. The humidity is a major determinant of potential

evapotranspiration. So, it determines the water

requirement of crops

5. High humidity reduces irrigation water

requirement of crops as the evapotranspiration

losses from crops depends on atmospheric

humidity

6. High humidity can prolong the survival of crops

under moisture stress. However, very high or very

low relative humidity is not conducive to higher

yields of crops

7. There are harmful effects of high humidity. It

enhances the growth of some saprophytic and

parasitic fungi, bacteria and pests, the growth of

which causes extensive damage to crop plants.

88

Eg: a. Blight disease on potato.

b. The damage caused by thrips and jassids on

several crops

8. High humidity at grain filling reduces the crop

yields

9. A very high relative humidity is beneficial to

maize, sorghum, sugarcane etc, while it is

harmful to crops like sunflower and tobacco

10. For almost all the crops, it is always safe to have

a moderate relative humidity of above 40%.

Variation in Humidity

1. Absolute humidity is highest at the equator and

minimum at the poles.

2. Absolute humidity is minimum at sunrise and

maximum in afternoon from 2 to 3 p.m. The diurnal

variations are small in desert regions.

3. The relative humidity is maximum at about the

sunrise and minimum between 2 to 3 p.m.

4. The behaviour of relative humidity differs a lot from

absolute humidity. At the equator it is at a maximum

of 80 per cent and around 85 per cent at the poles.

But, near horse latitudes it is around 70 per cent.

Measurement of Humidity

89

90

Wind

Air in horizontal motion is known as wind.

Vertical movement is noticed but negligibly small

compared to horizontal movement as the height of the

atmosphere is only for few kilometers. However vertical

movement or uplift of air only causes significant

weather changes in cloud formation and rain.

Wind systems of the world

The wind belts found on earth’s surface in each

hemisphere are:

a) Doldrums

b) Trade wind belt

c) Prevailing westerlies

d) Polar easterlies

1. Doldrums

Owing to continuous heating of the earth by insolation,

pressures are low and winds converge and rise near the

equator. This intertropical convergent zone is known as

‘Doldrums’.

a) These are the equatorial belts of calms and

variable winds.

b) The location is 5°S and 5°N latitudes.

c) Wind is light due to negligible pressure gradient.

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d) Mostly, there are vertical movements in the

atmosphere.

e) The atmosphere is hot and sticky.

2. Trade winds (Tropical Easterlies)

a) The regular high temperature at the equator

results in a high pressure forming in the upper

levels of the equator.

b) Then, the air is transferred to the northward and

southward directions until 35° North and South in

both the hemisphere.

92

c) Due to this reduction in surface pressure on the

equator (doldrums) there is an increase in

pressure at 35°N and 35°S which are known as

horse latitude (sub-tropical high).

d) As a result, the winds flow from the horse

latitude to the equatorial region.

e) While moving, these winds are deflected by

Coriolis force to the right in northern

hemisphere and to the left in southern

hemisphere.

f) These winds flow from 35°N to the equator in NE

direction in the northern hemisphere and from

35°S to the equator in SE direction in the

southern hemisphere. These are known as ‘Trade

winds’. These are known as ‘Tropical easterlies’.

g) These are most constant winds in force and

direction and flow over nearly half the globe.

3. Anti-trade winds

a) This is a supplementary wind system of the earth

which is effective at higher levels.

b) This system works in opposite direction to the

surface winds.

93

c) The anti-trade winds mostly flow from land to

ocean and brings no rain.

4. Prevailing Westerlies

a) The winds that flow from sub-tropical high to the

low-pressure area about 60-70° latitudes in both

the hemispheres are known as ‘Prevailing

westerlies’.

b) In the northern hemisphere the direction of

Prevailing westerlies is SW and in southern

hemisphere NW.

c) These winds are forceful and are irregular as

compare to the trade winds in the tropical

regions. This is high precipitation zone

5. Polar Easterlies / Polar winds

a) A permanent high pressure exists on the poles.

b) From these high pressure polar regions, cold

winds flow to areas at about 60-65° latitudes in

both the hemispheres.

c) The winds flow in NE direction in the northern

hemisphere and in SE direction in the southern

hemisphere.

Mountain winds

a) Blows from mountain up slope to base

94

b) Occurs during night time

c) Cooling of air close to slope takes place

d) Adiabatic heating decreases this phenomenon

e) Also known as ‘Katabalic winds’

Valley winds

a) Blow from valley base to up slope.

b) Occurs during day time

c) Over heating of air adjacent to slope takes place

d) Adiabatic cooling decreases this phenomenon

e) Also known as ‘Anabatic winds’

Sea breeze

During the daytime, more in summer, land is

heated more than the adjacent body of water. As a

result warmed air over the land expands producing an

area of low pressure. The isobaric surfaces bend

upward as a result of which the cooler air starts moving

across the coast line from sea to land. This is the ‘Sea

breeze; or ‘On shore breeze’.

Land breeze

At night because of nocturnal radiation land is

colder than adjacent sea and the pressure gradient is

directed from land to sea. There is a gentle flow of wind

95

from land to sea. This ‘off-share’ wind is called ‘Land

breeze’.

Effect of wind on crop plants

1. Transports heat in either sensible or latent heat form

from lower to higher altitudes

2. Wind affects the plant directly by increasing

transpiration and the intake of CO2 and also causes

several types of mechanical damage

3. Wind helps in pollination and dispersal of seeds

4. Light and gentle winds are helpful for cleaning the

agricultural produce

5. Hot dry winds frequently do much damage to

vegetation in the growing crops by promoting

excessive water loss

6. Wind has powerful effect on humidity

7. Long, continued warm, dry winds injured blossoms

by evaporating the secretion of the stigma

8. Provides moisture which is necessary for

precipitation

9. Wind prevents frost by disrupting atmospheric

inversion

10. Causes soil erosion

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Speed and Direction of wind

Measurement of wind

Chapter 4

Rainfall and Evaporation

Clouds

Clouds are condensed moisture consisting of

droplets of water and ice crystals. Clouds are defined as

“An aggregation of minute drops of water suspended in

the air at higher altitudes”. The rising air currents tend

to keep the clouds from falling to the ground.

Cloud formation

When air rises due to increase in temperature,

the pressure begins less it expands and cools until the

temperature is equalized. If the cooling proceeds

further till the saturation point, the water vapour

condenses and the cloud is formed.

Basic types of clouds

97

Cirrus (Ci) - means “curl” which is recognized by its

veil like fibrous or feathery form.

Cumulus (Cu) – means heaps or globular mass.

Stratus (St) - look like a sheet.

Nimbus (Nb) – Looks dark and ragged.

WMO cloud classification

The World Meteorological Organisation (WMO)

classified the clouds according to their height and

appearance into 10 categories. From the height, clouds

are grouped into 4 categories (viz., High clouds, Middle

clouds, low clouds and vertical clouds) as stated below

and there are sub- categories in each of these main

categories.

Family “A” or High clouds

The clouds in this category are high. The mean

lower level is 7 kilometers and the mean upper level is

12 kilometers in tropics and sub-tropics. In this family

there are three sub-categories.

1. Cirrus (Ci)

In these clouds ice crystals are present.

Looks like wispy and feathery. Delicate, desist,

white fibrous, and silky appearance.

98

Sun rays pass through these clouds and sunshine

without shadow.

Does not produce precipitation.

2. Cirrocumulus (Cc)

Like cirrus clouds ice crystals are present in these

clouds also.

Looks like rippled sand or waves of the sea shore.

White globular masses, transparent with no

shading effect.

Mackerel sky.

3. Cirrostraturs (Cs)

Like the above two clouds ice crystals are present in

these clouds also.

Looks like whitish veil and covers the entire sky with

milky white appearance.

Produces “Halo”.

Family “B” or Middle clouds

The clouds in this category are middle clouds. The

mean lower level is 2.5 kilometers and the mean upper

level is 7 kilometers in tropics and sub-tropics. In this

family there are 2 sub-categories as details below:

1. Altocumulus (Ac)

In these clouds ice water is present.

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Greyish or bluish globular masses.

Looks like sheep back and also known as flock

clouds or wool packed clouds.

2. Alto-stratus (As)

In these clouds water and ice are present separately.

Looks like fibrous veil or sheet and grey or bluish in

colour.

Produces coronos and cast shadow.

Rain occurs in middle and high latitudes.

Family “C” or Low clouds

The clouds in this category are lower clouds. The

height of these clouds extends from ground to upper

level of 2.5 kilometers in tropics and sub-tropics. In this

family, like high clouds, there are 3 sub-categorises.

1. Strato cumulus (Sc)

These clouds are composed of water.

Looks soft and grey, large globular masses and

darker than altocumuls.

Long parallel rolls pushed together or broken

masses.

The air is smooth above these clouds but strong

updrafts occur below.

2. Stratus (St)

100

These clouds are also composed of water.

Looks like for as these clouds resemble grayish

white sheet covering the entire portion of the sky

(cloud near the ground).

Mainly seen in winter season and occasional drizzle

occurs.

3. Nimbostratus (Ns)

These clouds are composed of water or ice crystals.

Looks thick dark, grey and uniform layer which

reduces the day light effectively.

Gives steady precipitation.

Sometimes looks like irregular, broken and

shapeless sheet like.

Family “D” or Vertical clouds

These clouds form due to vertical development i.e.,

due to convection. The mean low level is 0.5 and

means upper level goes up to 16 kilometers.

In this family two sub-categories are present.

1. Cumulus (Cu)

These clouds are composed of water with white

majestic appearance with flat base.

101

Irregular dome shaped and looks like cauliflower

with wool pack and dark appearance below due to

shadow.

These clouds usually develop into cumuolo-nimbus

clouds with flat base.

2. Cumulonimbus (Cb)

The upper levels of these clouds possess ice and

water is present at the lower levels.

These clouds have thunder head with towering

evil top and develop vertically.

These clouds produces violent winds, thunder

storms, hails and lightening, during summer.

Precipitation

Precipitation is defined as water in liquid or solid form

falling on the earth’s surface.

Isohyets: Isohyets are the lines connecting various

locations, having an equal amount of precipitation.

Types of precipitation

1. Cyclonic precipitation

2. Conventional precipitation

3. Orographic precipitation

4. Precipitation due to turbulent ascent

1. Cyclonic precipitation

102

Cyclonic precipitation results from lifting of air masses

converging into low pressure area of cyclones.

2.Conventional precipitation

This type of precipitation occurs due to

convection over turning of moist air. Conventional

precipitation results in heavy showery rainfall. The

major forms of precipitation associated with this type

are rains or snow showers, hail or snow pellets.

3. Orographic precipitation

Precipitation resulting from raising and cooling of

air masses when they are blocked by a topographical

barrier (mountains). The barriers are important factor in

increasing the rainfall on windward slopes.

4. Precipitation due to turbulent ascent

It is produced when airs current converge and rise.

Most precipitation results from condensation and

sublimation. This type occurs mainly in middle latitude.

a) Rainfall intensity

b) Aerial extent of rainstorm

c) Frequency of rainstorms

Forms of Precipitation

Liquid form

Rain

103

It is precipitation of liquid water particles either

in the form of drops having diameter greater than 0.5

mm or in the form of smaller widely scattered drops.

When the precipitation process is very active, the lower

air is moist and the clouds are very deep, rainfall is in

the form of heavy downpour. On occasions, falling

raindrops completely evaporate before reaching the

ground.

Drizzle

It is fairly uniform precipitation composed of fine

drops of water having diameter less than 0.5 mm small

and uniform size and seems to be floated in the air, it is

referred as drizzle. If the drops in a drizzle completely

evaporates before reaching the ground, the condition is

referred to as ‘mist’.

Shower

Solid form

Snow

It is the precipitation of white and opaque grains

of ice. Snow is the precipitation of solid water mainly in

the form of branched hexagonal crystals of stars. In

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winter, when temperatures are below freezing in the

whole atmosphere, the ice crystals falling from the

Altostratus do not melt and reach the ground as snow.

Hail

Precipitation of small pieces of ice with diameter

ranging from 5 to 50 mm or something more is known

as hail. Hailstorms are frequent in tropics. In India, the

period from March to May offers the ideal condition for

hailstorms. It is the most dreaded and destructive form

of precipitation produced in thunderstorms or

cumulonimbus clouds.

Mixed form

Sleet

It refers to precipitation in the form of a mixture of

rain and snow. It consists of small pellets of transparent

ice, 5 mm or less in diameter. It refers to a frozen rain

that forms when rain falling to the earth passing

through a layer of cold air and freezes. This happens

when temperature is very low. It is not commonly seen

in India expect high ranges, that too in winter, in

extreme north and northeast India.

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Glaze

Freezing rain is known as glaze. This is formed at sub-

freezing temperature, when rain falls on objects or on

ground. It looks like a sheet or coat.

Condensation

Hydrological cycle

The water cycle also known as the hydrological

cycle, describes the continuous movement of water on

above and below the surface of the earth. There is no

beginning or end. Water can change states among

liquid, vapour and ice at various places in the water

cycle. Although the balance of water on earth remains

fairly constant over time, individual water molecules

can come and go. It consists of four major steps viz.,

Evaporation, transportation, condensation and

precipitation.

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Monsoons of India

The term monsoon is derived from an Arabic word

“Mausim” means “Season”.

Indian continent receives its annual rainfall by the

peculiar phenomenon known as monsoon. It consists of

series of cyclones that arise in India Ocean. These

travel in northeast direction and enter the Peninsular

India along its west coast. The most important of these

cyclones usually occur from June to September

resulting in southwest monsoon. Southwest monsoon is

the most important as it contribute 80 – 95% of the

total rainfall of the country.

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Two types of monsoon systems are a) South

West Monsoon, b) North East Monsoon.

(a)South West Monsoon

Beginning of the year temperature of the Indian

Peninsular rapidly rises under the increasing heat of the

sun. A minimum barometric pressure is established in

the interior parts of the Peninsular by the month of

March. Westerly winds prevail on the west Kerala and

south winds on the west of northern Circars, Orissa and

Bengal. During April and May the region of high

temperature is shifted to north viz., upper Sind, lower

Punjab and Western Rajasthan. This area becomes the

minimum barometric pressure area to which monsoon

winds are directed.

The western branch of South West monsoon

touches North Karnataka, Southern Maharastra and

then it make its way to Gujarat. When the South West

Monsoon is fully operating on the Western India,

another branch of the same is acting in the Bay of

Bengal. It carries rains to Burma, Northern portions of

the east coast of India, Bengal, Assam and the whole of

North India in general.

b) North east Monsoon

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During September end, the South West Monsoon

penetrates to North Western India but stays on for a full

month in Bengal. On account of the increase in

barometric pressure in Northern India, there is a shift of

the barometric pressure to the South East and North

Easterly winds begin to flow on the eastern coast.

These changes bring on heavy and continuous rainfall

to the Southern and South Eastern India.

c) Winter Rainfall

It is restricted more to Northern India and is received in

the form of snow on the hills and as rains in the plains

of Punjab, Rajasthan and central India. Western

disturbance is a dominant factor for rainfall during

these months in northwestern India.

d) Summer Rainfall:

The summer Rainfall is received from March to

May as local storms. It is mostly received in the South

East of Peninsular and in Bengal. Western India does

not generally receive these rains.

Rainfall distribution in Tamilnadu in different

seasons

Rainfall

season

Quantity

(mm)

Percentage

share

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South West

monsoon

311.7 32

North East

Monsoon

457.8 47

Cold weather

period

48.7 5

Hot weather

period

155.9 16

Effect of monsoon on crop production

1. Nearly 54 per cent of population of the world

depends on monsoon for their income.

2. Monsoon rains are considered as life giving rains.

3. Heavy rain during harvesting causes lodging of crop

and seed germination.

4. As in the case of other weather elements the

amount and distribution of rainfall influence the

crop yield considerably.

5. Many farm operations such as seed bed

preparation, sowing, intercultivation etc., depend on

rainfall.

Harmful effect of rainfall

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1. When heavy rainfall occurs sowing, weeding,

harvesting and drying becomes very difficult.

2. Rainfall received immediately after harvesting

causes germination of seeds and growth of fungal

population

3. If low rainfall occurs, it results in non- supply of

sufficient water to the plant which finally results in

drying up.

Artificial rain making

Clouds are classified into warm and cold clouds

based on cloud top temperature. If the cloud

temperature is positive these clouds are called warm

clouds and if it is negative they are called as cold

clouds. The nucleus needed for precipitation differs with

type of clouds. Hygroscopic materials are necessary as

nucleus for warm clouds

Cloud seeding

Cloud seeding is one of the tools to mitigate the

effects of drought. It is defined as a process in which

the precipitation is encouraged by injecting artificial

condensation nuclei through aircrafts or suitable

mechanism to induce rain from rain bearing cloud. The

rain drops are several times heavier than cloud

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droplets. These mechanisms are different for cold and

warm clouds.

Seeding of cold clouds

This can be achieved by two ways 1. Dry ice seeding

and 2. Silver Iodide seeding

1. Dry ice seeding

Dry ice (solid carbon-dioxide) has certain specific

features. It remains as it is at –80°C and evaporates,

but does not melt. Dry ice is heavy and falls rapidly

from top of cloud and has no persistent effects due to

cloud seeding.

Aircrafts are commonly used for cloud seeding with dry

ice.

Aircraft flies across the top of a cloud and 0.5 – 1.0 cm

dry ice pellets are released in a steady stream.

While falling through the cloud a sheet of ice crystals is

formed.

From these ice crystals rain occurs.

This method is not economical as 250 kg of dry

ice is required for seeding one cloud. To carry the

heavy dry ice over the top of clouds special aircrafts

are required, which is an expensive process.

2. Silver Iodide seeding

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Minute crystals of silver iodide produced in the

form of smoke acts as efficient ice-farming nuclei at

temperatures below –5°C. When these nuclei are

produced from the ground generators, these particles

are fine enough to diffuse with air currents. Silver

iodide is the most effective nucleating substance

because; its atomic arrangement is similar to that of

ice. The time for silver iodide smoke released from

ground generator to reach the super cooled clouds was

offer some hours, during which it would draft a long

way and decay under the sun light. The appropriate

procedure for seeding cold clouds would be to release

silver iodide smoke into super cooled cloud from an

aircraft. In seeding cold clouds silver iodide technique is

more useful than dry ice techniques, because, very

much less of silver iodide is required per cloud. There is

no necessity to fly to the top of the cloud, if area to be

covered is large.

Seeding of warm clouds

1) Water drop Technique

2) Common salt technique

Evaporation

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A physical process in which liquid water is

converted into its vapour. Evaporation is the most

important water loss term in water balance equation.

Importance of Evaporation in crop plants

1) Evaporation is an important process of

hydrologic cycle.

2) The evaporation from the soil is an important

factor deciding the irrigation water requirements of

a crop

3) In modifying the microclimate of a crop the

evaporation from the soils is an important factor for

consideration.

4) Evaporation is the most important of all the

factors in the heat budget, after radiation.

5) The evaporation is also one of the most

important factors in the water economy.

6) Since, a certain amount of evaporation also

demands a definite amount of heat, it provides a

link between water budget and heat budget.

Factors affecting evaporation

The evaporation from a fully exposed water surface

is the function of several environmental factors

1. Environmental factors

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a. Water temperature

With an increase of temperature the kinetic energy

of water molecules increases and surface tension

decreases which increases evaporation.

b. Wind

The evaporation from fully exposed surface is

directly proportional to the velocity of wind and

vice-versa, because dry wind replaces the moist air

near water. The process of evaporation takes place

continuously when there is a supply of energy to

provide latent heat of evaporation (540 calories /

gram of water).

c. Relative humidity

The evaporation is greater at low RH than at

high RH.

d. Pressure

The evaporation is more at low pressure and

less at high pressure.

2. Water factors

a. Composition of water

The dissolved salts and other impurities decreases

the rate of evaporation. The evaporation is inversely

proportional to the salinity of water.

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b. Area of evaporation

The larger the area of exposure, greater will

be the evaporation.

Transpiration

This is a physiological phenomenon, which takes

place only in living plants. The loss of water from living

parts of the plant is known as Transpiration. The loss of

water through stomatal openings of the leaves is

termed as stomatal transpiration. The loss of water

through cuticle is known as cuticular transpiration and

from lenticels is known as lenticular transpiration.

Importance of transpiration on crop plants

1. Dissipation of radiant energy by plant parts

2. Translocation of water in the plants

3. Translocation of minerals in the plant

Factors affecting transpiration

I. Environmental factors

1. Light: By directly opening and closing

the of stomata there is periodicity in the transpiration

rate. Indirectly by increasing the temperature of leaf

cells the transpiration is increased.

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2. Atmospheric humidity: The rate of

transpiration is almost inversely proportional to

atmospheric humidity.

3. Air Temperature: Increase in

Temperature results in opening of stomata which in

turn increases transpiration.

4. Wind velocity: The higher the wind

speed higher the transpiration

II. Plant factors

1. Plant height: Water need of the crop

varies with height.

2. Leaf characteristics: Reduction in leaf

area brings reduction in transpiration.

3. Availability of water to the plant: If

there is little water in the soil the tendency for

dehydration of leaf causes stomatal closure and a

consequent fall in transpiration.

Evapotranspiration

Evapotranspiration denotes the quantity of water

transpired by plants or retained in the plant tissue plus

the moisture evaporated from the surface of the soil. As

long as the rate of root uptake of soil moisture balances

the water losses from the canopy, evapotranspiration

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continues to occur at its potential rate. When the rate

of root water uptake falls below the transpiration

demand, actual transpiration begins to fall below the

potential rate. This is either because the soil cannot

supply water to roots quickly or the plant can no longer

extract water to meet the evaporational demand.

Reference Evapotranspiration (ET0)

This represents the maximum rate of

evapotranspiration from an extended surface of 8 to 10

centimeters tall green grass cover, actually growing

and completely shading the ground under limited

supply of water.

Potential Evapotranspiration (PET) is the upper

limit of evapotranspiration. PET is the water transpired

from uniform, short, green, actively growing vegetation

when water supply is unlimited. ET data are used as a

basis for estimating the amount of water required to

irrigate.

Lysimeter is a device for measuring ET. There

are two types of Lysimeter, they are namely

volumetric, which is used for paddy, and the other is

gravimetric which is used in dry land and irrigated

crop like maize, cotton and groundnut.

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Importance of Evapotranspiration and Potential

Evapotranspiration for crop plants

1. Estimation of the soil moisture there by planning

irrigation schedule of crops.

2. Understanding relationship between the crop

yield and irrigation water.

3. Guiding for the production of a crop with a fully

developed canopy.

4. The evapotranspiration can also help to

demarcate soil climatic zones including the

drought prone areas.

5. These will form the base for developing suitable

soil and crop management practices, crop

varieties, water conservation techniques,

cropping pattern and ways to improve

productivity rainfed crops.

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Chapter 5

Weather Aberrations and Forecasting

Weather Aberrations

Weather is not always normal. Some abrupt or

sudden changes take place in weather which is called

weather abnormalities. They cause considerable

damage to the crops. Some of them are droughts,

floods, untimely rains, thunder storms, hail storms, dust

storms and cold waves.

Drought or deficient rainfall

The term drought can be defined in several ways.

1. The condition under which crops fail to mature

because of insufficient supply of water through rain.

2. The situation in which the amount of water

required for transpiration and evaporation by crop

plants in a defined area exceeds the amount of

available moisture in the soil.

3. A situation of no precipitation in a rainy season

for more than 15 days continuously. Such length of

non-rainy days can also be called as dry spells.

Classification of Drought

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Droughts are broadly divided into 3 categories

based on the nature of impact and spatial extent.

1. Meteorological Drought

If annual rainfall is significantly short of certain

level (75 per cent) of the climatologically expected

normal rainfall over a wide area, then the situation is

called meteorological drought. In every state each

region receives certain amount of normal rainfall. This

is the basis for planning the cropping pattern of that

region or area.

2. Hydrological drought

This is a situation in which the hydrological

resources like streams, rivers, reservoirs, lakes, wells

etc., dry up because of marked depletion of surface

water. The ground water table also depletes. The

industry, power generation and other income

generating major sources are affected. If Meteorological

drought is significantly prolonged, the hydrological

drought sets in.

3. Agricultural Drought

This is a situation, which is a result of inadequate

rainfall and followed by soil moisture deficit. As a result,

the soil moisture falls short to meet the demands of the

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crops during its growth. Since, the soil moisture

available to a crop is insufficient, it affects growth and

finally results in the reduction of yield. This is further

classified as early season drought, mid-season drought

and late season drought.

Flood

Years in which actual rainfall is ‘above’ the

normal by twice the mean deviation or more is defined

as years of flood or excessive rainfall. Like droughts,

the definition of floods also varies one situation to

another and forms one region to other. 2005 is the

heavy flood year in Tamil nadu.

Cyclone

Cyclones are most common in India and cause

heavy damage to crops, livestock and loss of human

life. Tropical cyclones as the name implies, are cyclonic

whirlpools that are formed over the oceans in the

latitudes between 5 and 20 degree from the equator.

These storms are called by different local names, the

most common being hurricane (North America),

typhoon (Eastern Asia, Philippines, Cyclone (India,

Australia).

Tornado

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Tornado is the most violent storms of lower

troposphere. They mostly occur during spring and early

summer. Crop losses are heavy due to this event.

Thunder storm

Thunderstorms occur regularly in the tropics.

Thunder and lightning accompany the typical mature

thunderstorm. Raindrops are large and it is literally a

“cloud burst”. Thunderstorms are most common in

summer due to surface heating of land mass.

Dust storms

Dust storms take place during summer in the

months of April, May and June. During this period, high

temperature and low humidity prevail. Dust and

particles may deposit on the crops.

Effect of weather aberrations on agriculture:

1. The crops fail to grow and mature because of

insufficient availability of moisture.

2. Due to submergence due to muddy water, the fields

are silted.

3. Due to higher rainfall tank bunds are breached.

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4. Floods cause damage to crops, livestock and human

life.

5. Cyclones cause damage either by high wind or by

flooding.

Phenology

Phenology is the study of the annual cycles of

plants and how they respond to seasonal changes in

their environment. For example, in botany phenology

refers to the timing of flower emergence, sequence of

bloom, fruiting, and leaf drop in autumn.

Uses of Phenology

Correlation with insect emergence and pest

control

Correlation with crop planting dates

Farmscaping with insect refugia (cover crops,

hedge rows, strip crops) to attract beneficial

insects and enhance natural biological control

Designing orchards for pollination and ripening

sequence

Designing bee forage plantings

Designing perennial flower beds and wildflower

plantings

Prediction of global warming trends

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Bioclimatic law

The law which states that phenological events are

altered by about 4 days for each 5° change of latitude

northward or longitude eastward; events are

accelerated in spring and retreat in autumn.

Different season of India

Based on the rainfall pattern and temperature

distribution, the India Meteorological Department,

Government of India has divided the whole year into

four seasons viz.

1. South west monsoon (June – September)

2. Post Monsoon (October – November)

3. Winter (December – February)

4. Summer or Pre monsoon (March / April – May /

June)

The above seasons coincide with the agricultural

seasons viz.

1) Kharif

2) Rabi and

3) Summer

The Kharif season is nothing but the Southwest

monsoon or autumn. The Rabi coincides with post

monsoon and winter seasons. In summer, the cultivable

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land under seasonal crops is kept in many regions of

the country. Wherever water is plenty, vegetables are

grown in some parts of the country during summer.

Spring falls between January and March. Based on

temperatures, ranges, there are three distinct crops

seasons in India. They are

1) Hot weather (Mid February – Mid June)

2) Kharif or rainy season (Mid June – Mid October)

3) Rabi (Mid October – Mid February)

In Tamil Nadu there is a slight variation in the seasons

based on rainfall duration as

2) Winter – January and February

3) Summer – March to May

4) Rainy seasons

a) South west monsoon (June – September)

b) North east monsoon (October – December)

The criteria for division of growing seasons are broadly

based on monthly precipitation and temperature. The

pattern followed as

a) Hot month – if the average temperature is

above 20°C

b) Cold month – if the mean temperature is

between 0-10°C

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c) Warm month – if the mean temperature is

15-20°C

MAPPING OF AGROCLIMATIC ZONES OF INDIAAND

TAMIL NADU CLIMATE

Climate in general is the totality of weather

during a large period and wider areas.

AGRO CLIMATE

Agro climate can be defined as the conditions and

the effect of varying parameters like solar radiation,

rainfall, relative humidity on crop growth and

production. The climate of one place is not similar to

other area.

However identical climatic conditions are prevalent

in regions of the same latitude.

ADVANTAGES OF AGRO CLIMATIC ZONES

1. This would enable in exploring agricultural

potentiality of the area.

2. Location of homo climatic zones will enable in

identifying the soil and climate problems related to

agriculture to each region.

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3. This will help in introduction of new crops from other

similar area. Eg. Introduction of oil palm in

Tamil Nadu.

4. Development of crop production technologies

specific for the region.

5. Take a research to solve the regional problems.

6. To transfer the technology easily.

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A. AGRO CLIMATIC ZONES OF INDIA

Based on the criteria of homogeneity in agro-

characteristics such as rainfall, temperature, soil,

topography, cropping, farming systems and water

resources, the country has been divided into 15 agro-

climatic zones. This was mainly done to identify a

production constraints and to plan future strategies for

purpose of cropping with the involvement of

academists, scientists, planners, administrators,

voluntary agencies, bank and others for balanced

regional growth.

1. WESTERN HIMALAYAN ZONE

This zone consists of three distinct sub-zones of

Jammu and Kashmir, Himachal Pradesh and Uttar

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Pradesh Hills. The region consists of skeletal soils of

cold region, podsolic soils, mountain meadow soils, hilly

brown soils. Lands of the region have steep slopes in

undulating terrain. Soils are generally silty loams and

these are prone to erosion hazards.

2. EASTERN HIMALAYAN ZONE

Sikkim and Darjeeling Hills, Arunachal Pradesh,

Meghalaya, Nagaland, Manipur, Tripura, Mizoram,

Assam and Jalpaiguri and Coochbihar districts of West

Bengal fall under this region, with high rainfall and high

forest cover. Shifting cultivation is practiced in nearly

one-third of the cultivated area and this has caused

denudation and degradation of soils with the resultant

heavy runoff, massive soil erosion and floods in the

lower reaches and basins.

3. LOWER GANGETIC PLAINS ZONE

This zone consists of West Bengal-lower Gangetic

plains region. The soils are mostly alluvial and are

prone to floods.

4. MIDDLE GANGETIC PLAINS

This zone consists of 12 districts of eastern Uttar

Pradesh and 27 districts of Bihar plains. This zone has

a geographical area of 16 million hectares and rainfall

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is high. About 39 per cent of gross cropped area is

irrigated and the cropping intensity is 142 per cent.

5. UPPER GANGETIC PLAINS ZONE

This zone consists of 32 districts of Uttar

Pradesh. Irrigation is through canals and tube wells. A

good potential for exploitation of ground water exists.

6. TRANS GENETIC PLAINS ZONE

This zone consists of Punjab, Haryana, Union

Territories of Delhi and Chandigarh and Sriganganagar

district of Rajasthan. The major characteristics of this

area are highest net sown area, highest irrigated area,

high cropping intensity and high groundwater

utilization.

7. EASTERN PLATEAU AND HILLS ZONE

This zone consists of eastern part of Madhya

Pradesh, southern part of West Bengal and most of

inland Orissa. The soils are shallow and medium in

depth and the topography is undulating with a slope of

one to ten per cent. Irrigation is through tanks and

tube wells.

8. CENTRAL PLATEAU AND HILLS

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This zone comprises 46 districts of Uttar Pradesh,

Madhya Pradesh and Rajasthan. Nearly ⅓rd of land is not

available for cultivation.

9. WESTERN PLATEAU AND HILLS ZONE

This zone comprises the major part of

Maharashtra, parts of Madhya Pradesh and one district

of Rajasthan. The average rainfall of the zone is 904

mm. The net sown area is 65 per cent and forests

occupy 11 per cent. The irrigated area is only 12.4 per

cent with canals being the main source.

10. SOUTHERN PLATEAU AND HILLS

This zone comprises 35 districts of Andhra

Pradesh, Karnataka and Tamil Nadu which are typically

semi-arid zones. Dryland farming is adopted in 81 per

cent of the area and the cropping intensity is 111 per

cent.

11. EAST COAST PLAINS AND HILLS

This zone comprises west coast of Tamil Nadu,

Andhra Pradesh and Orissa. Soils are mainly alluvial

and coastal sands. Irrigation is through canals and

tanks.

12. WEST COAST PLAINS AND GHATS ZONE

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This zone comprises west coast of Tamil Nadu,

Kerala, Karnataka, Maharashtra and Goa with a variety

of crop patterns, rainfall and soil types.

13. GUJARAT PLAINS AND HILLS ZONE

This zone consists of 19 districts of Gujarat. This

zone is arid with low rainfall in most parts and only 32.5

per cent of the area is irrigated largely through wells

and tubewells.

14. WESTERN DRY ZONE

This zone comprises nine districts of Rajastan

and is charecterised by hot sandy desert, erratic

rainfall, high evaporation, scanty vegetation. The

ground water is deep and often brackish. Famine and

drought are common features of the region.

15. ISLAND ZONE

This zone converts the island territories of

Andaman and Nicobar and Lakshadeep which are

typically equatorial with rainfall of 3000 mm spread

over eight to nine months. It is largely a forest zone

with undulated lands.

B. AGRO CLIMATIC ZONES OF TAMIL NADU

Tamil Nadu recorded the mean annual rainfall of

974 mm. The highest contribution from north-east (NE)

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monsoon (47 per cent of total rainfall), followed by

south-west (SE) monsoon (32 per cent), summer (16

per cent) and winter

(5 per cent). Based on the rainfall, altitude and

irrigation source, the state has been marked into seven

agroclimatic zones.

1. North – Eastern Zone

Thiruvallur, Vellore, Kanchipuram,

Thiruvannamalai, Villupuram and Cuddalore district.

The mean annual rainfall is 1100 mm out of which 566

mm is received during the NE monsoon period with

frequent occurrence of cyclones. The major soil types

are red sandy loam, clayey loam and saline coastal

alluvium. The major irrigation sources are tanks and

wells. Since the rainfall is uniformly distributed from

July to December, two crop sequences are practiced. In

the coastal areas rainfed rice is cultivated. The crops

like sugarcane and millets are raised with the help of

well irrigation.

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Agro Climatic zones of Tamil Nadu

2. North – Western Zone

Salem, Dharmapuri, Krishnagiri and Namakkal

(part) districts. The mean annual rainfall is 875 mm and

about 42 per cent of rainfall is received during the SW

monsoon period. The elevation ranges from 800 to

1000 m (MSL). The major soils are non-calcareous red

and brown and calcareous black. Cultivation in

drylands commences from June and two crop

sequences are followed. A significant practice of

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transplanting finger millet, sorghum and tomato purely

under rainfed condition is followed. Established mango

grooves in drylands are common.

3. Western Zone

Coimbatore, Erode, parts of Karur, Namakkal,

Dindigul and Theni districts. The mean annual rainfall is

715 mm with a contribution of 49 per cent from the NE

period. The predominant soil types are red and black

soils. Dryland sowings start in June / July in red soils

while in black soils during September / October. With

the receipt of early rain, groundnut is sown in red soils.

In black soil areas, cotton for early rains and

bengalgram for late rains are raised. Southern part of

this zone, the rainfall is about 550 mm only and more

area is devoted to pastures. With the help of well and

canal irrigation crops like cotton, finger millet and

sugarcane are raised.

4. Cauvery Delta Zone

Thanjavur, Tiruchirapalli, Thiruvarur,

Nagapattinam, Perambalur, parts of Pudukottai and

Cuddalore districts. The mean annual rainfall varies

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from 985 mm, out of which more than 50 per cent is

received through NE monsoon period. The main source

of irrigation is the Cauvery river. The major soil type is

alluvial in the old delta areas while red loamy and

lateritic are also present in other areas. Rice occupies

the major area and it is called as ‘rice bowl’ of Tamil

Nadu. After the rice crop, pulses are raised with

residual soil moisture. In places with supplemental

irrigation through filter points cotton, groundnut and

sesamum are raised as summer crops.

5. Southern Zone

Madurai, Sivagangai, Virudhunagar,

Ramanathapuram, Thoothukudi and Tirunelveli

districts. The topography of the zone is undulating.

This zone lies on the rain shadow area of the western

ghats. The mean annual rainfall is 850 mm with a

contribution of about 470 mm from NE monsoon. The

soils of this region fall under major groups, viz., black,

red, alluvial and lateritic. Saline coastal alluvial soils

are also present in the coastal belt. In black soils only

one crop, either cotton or sorghum is raised. Direct

seeded rice is cultivated under rainfed condition on

light soils. On red soils, groundnut crop is raised.

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Under garden land conditions, pearl millet and chillies

are raised.

6. High Rainfall Zone

Kanyakumari district -The mean annual rainfall is

1420 mm received in 64 rainy days, out of which 38

and 36 per cent are respectively received during SW

and NE monsoon periods. The soils are deep red loam

except the coastal areas where it is saline coastal

alluvium. Rice is the major annual crop under rainfed

condition followed by tapioca. Plantation crops like tea,

pepper, clove, nutmeg, cardamom and coffee also

cultivated on the hills.

7. Hilly Zone

This zone comprises the hilly region of the

Nilgiris, the Shevroys, the Yalagiri, the Anamalai and

the Palani. The mean annual rainfall is 2124 mm. The

soil is mainly lateritic. The major crops are cole

vegetables, potato, tropical and temperate fruits. At

the foot of the hills minor millets are raised. At higher

altitudes wheat cultivation is common during winter

season.

Weather forecast

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The prediction of weather for the next few days

to follow. The Figure below depicts different weather

forecasting services normally practiced in a country.

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Weather forecasting

services

Agriculture including forestry

and Animal husbandryGeneral

Public

Fishing

Mountaineering

Cyclones, floods and

drought

Government and Post officials

Off shore drilling

Aviation Civil & Military

Defence services

Shipping Mercantile & Naval

NEED / IMPORTANCE OF FORECAST

Basically weather has many social and economic

impacts in a place.

Among different factors that influence crop

production, weather plays a decisive role as

aberrations in it alone explains up to 50 per cent

variations in crop production

The rainfall is the most important among the

required forecast, which decides the crop

production in a region and ultimately the

country’s economy.

The planning for moisture conservation under

weak monsoon condition and for flood relief

under strong monsoon condition is important in a

region.

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A reliable weather forecasting when

disseminated appropriately will pave way for the

effective sustainability.

One can minimize the damage, which may be

caused directly or indirectly by unfavourable

weather.

The recurring crop losses can be minimized if

reliable forecast on incidence of pest and

diseases is given timely based on weather

variables.

Help in holding the food grain prices in check

through buffer stock operations. This means that

in good monsoon years when prices fall, the

government may step in and buy and in bad

years when price tend to rise, it may unload a

part of what it had purchased.

Judicious use of water can be planned in a region

depending up on the forecast.

Type of weather forecast

Types of forecast

Validity period

Main users

Predictions

1 Short range a) Now

Up to 72 hours0-2 hours

Farmers marine agencies,

Rainfall distribution, heavy rainfall,

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casting general public

heat and cold wave conditions, thunder storms etc.

b) Very short range

0-12 hours

2 Medium range

Beyond 3 days and upto 10 days.

Farmers Occurrence of rainfall, Temperature.

3 Long range

Beyond 10 days upto a month and a season.

Planners This forecasting is provided for Indian monsoon rainfall. The out looks are usually expressed in the form of expected deviation from normal condition.

Synoptic charts

An enormous volume of meteorological data is

being collected from all over the world continuously

round the clock through various telecommunication

channels. To assess, assimilate and analyse the vast

data, they have to be suitably presented. For this

purpose, the observations are plotted on maps in

standard weather codes. These maps are called

‘Synoptic maps or charts’.

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Synoptic charts display the weather conditions at

a specified time over a large geographical area. The

surface synoptic charts plotted for different synoptic

hours (00, 03, 06, 09, 12, 15, 18, 21 UTC) depict the

distribution of pressure, temperature, dew point,

clouds, winds, present and past weather. In place of

GMT, UTC (Universal Time Co-ordinate) is used. The

upper air charts are also prepared at the standard

pressure levels of the atmosphere (different heights) of

the atmosphere wherein the pressure, wind and

temperature are plotted. The surface charts together

with the upper air charts provide a composite three-

dimensional weather picture pertaining to a given time.

Thus it gives a birds eye view of the state of

atmosphere at a time over a large area and is a

important tool used by operational meteorologists and

scientists.

The surface synoptic charts are the most used

charts. It contains the maximum number of

observations with the largest number of parameters

plotted and often forms the base on which the pressure

level charts are built up. The pattern of the pressure

distribution is brought out by drawing isobars, troughs,

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ridges, lows, highs, depressions, cyclones, cols, fronts

and discontinuities. These systems are clearly marked

and labeled using appropriate symbols and colours.

In synoptic charts different weather phenomena

and atmospheric characters are marked with different

symbols as mentioned below.

________________________________________________________

___________________

S.No Symbols

Weather

element/character/phenomenon

1. Narrow black lines lsobars

2. Numbers at ends of isobars

Pressure values in hPa

3. Shading

Precipitation

4. Arrows Wind

direction

5. Feathers in the arrows Wind

velocity

6. Small circles with shading Amount

of clouds

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In addition to the above, different symbols are used for

recording weather phenomena.

Crop Weather calendar

145

In order to provide the farmers with an efficient

weather service, it is essential that the weather

forecaster should be familiar with the crops that are

grown in a particular agroclimatic zone. The type of

forewarnings to be given depends on the stages of the

crop. In case of farmers, they should become familiar

with weather bulletins and learn how to interpret. To

meet the above requirement, the detailed information

collected from the agricultural departments has been

condensed by the IMD and presented in a pictorial form

known as crop weather calendar. This calendar has

three parts as follows.

a) Bottom part

b) Middle part

c) Top part

a) Bottom part provides the activities related to crop or

information related to phenological stages of the crop

and the months.

b) Middle part gives information regarding normal

weather condition required for active crop growth. It is

divided into different sections according to rainfall,

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rainy days, minimum temperature, maximum

temperature, pan evaporation and sunshine hours.

c) Top part gives information related to the weather

abnormalities or to take precautionary measures. Top

part is divided into different sections according to dry

spell length, high wind, heavy rainfall and cloudy

weather.

Sample crop weather calendar prepared for cotton

in Tamilnadu for South Arcot district

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148

REMOTE SENSING AND CROP MODELING

Remote Sensing is the science and art of

acquiring information (spectral, spatial, temporal) about

material objects, area, or phenomenon, without coming

into physical contact with the objects, or area, or

phenomenon under investigation. Without direct

contact, some means of transferring information

through space must be utilised. In remote sensing,

information transfer is accomplished by use of

electromagnetic radiation (EMR).

Uses: Remote sensing techniques are used in

agricultural and allied fields.

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1. Collection of basic data for monitoring of crop

growth

2. Estimating the cropped area

3. Forecasting the crop production

4. Mapping of wastelands

5. Drought monitoring and its assessment

6. Flood mapping and damage assessment

7. Land use/cover mapping and area under forest

coverage

8. Soil mapping

9. Assessing soil moisture condition, irrigation,

drainage

10. Assessing outbreak of pest and disease

11. Ground water exploration

Remote Sensing platforms

Three platforms are generally used for remote sensing

techniques. They are ground based, air based and

satellite based. Infrared thermometer, Spectral

radiometer, Pilot-Balloons and Radars are some of the

ground based remote sensing tools while aircrafts air

based remote sensing tools. Since the ground based

150

and air based platforms are very costly and have

limited use, space based satellite technology has

become handy for wider application of remote sensing

techniques. The digital image processing, using

powerful computers, is the key tool for analyzing and

interpretation of remotely sensed data. The advantages

of satellite remote sensing are:

Synoptic view – Wide area can be covered by a

single image/photo (One scene of Indian Remote

Sensing Satellite IRS series cover about 148 x 178

sq.km area).

Receptivity – Can get the data of any area

repeatedly (IRS series cover the same area every

16-22 days).

Coverage – Inaccessible areas like mountains,

swampy areas and thick forests are easily covered.

Space based remote sensing is the process of

obtaining information about the earth from the

instruments mounted on the Earth Observation

Satellites. The satellites are subdivided into two classes

and the types of satellite are as follows:

Polar orbiting satellites

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These satellites operate at an altitude between

550 and 1,600 km along an inclined circular plane over

the poles. These satellites are used for remote sensing

purposes. LANDSAT (USA), SPOT (FRANCE), and IRS

(INDIA) are some of the Remote Sensing Satellites.

Geostationary satellite

These have orbits around the equator at an

altitude of 36,000 km and move with the same speed

as the earth so as to view the same area on the earth

continuously. They are used for telecommunication and

weather forecasting purposes. INSAT series are

launched from India for the above purposes. All these

satellites have sensors on board operating in the visible

and near infrared regions of the electromagnetic

spectrum. INSAT-3A was launched on 10th April, 2003.

Role of Remote Sensing in agriculture

Agricultural resources are important renewable

dynamic natural resources. In India, agriculture sector

alone sustains the livelihood of around 70 percent of the

population and contributes nearly 35 percent of the net

national product. Increasing agricultural productivity has

been the main concern since scope for increasing area

under agriculture is rather limited. This demands judicious

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and optimal management of both land and water

resources. Hence, comprehensive and reliable information

on land use/cover, forest area, soils, geological

information, extent of wastelands, agricultural crops,

water resources both surface and underground and

hazards/natural calamities like drought and floods is

required. Season-wise information on crops, their acreage,

vigour and production enables the country to adopt

suitable measures to meet shortages, if any, and

implement proper support and procurement policies.

Remote Sensing systems, having capability of providing

regular, synoptic, multi-temporal and multi-spectral

coverage of the country, are playing an important role in

providing such information. A large number of

experiments have been carried out in developing

techniques for extracting agriculture related information

from ground borne, air borne and space borne data.

Indian Remote Sensing programme

India, with the experience gained from its

experimental remote sensing satellite missions

BHASKARA-I and II, has now established satellite based

operational remote sensing system in the country with

the launch of Indian Remote Sensing Satellite IRS-IA in

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1988, followed by IRS-IB (1992), IRS-IC (1995) and IRS-

ID (1997). The Department of Space (DOS) / Indian

Space Research Organisation (ISRO) as the nodal

agency for establishing an operation remote sensing

system in the country initiated efforts in the early

1970s for assessing the potentials of remotely sensed

data through several means. In order to meet the user

requirement of remote sensing data analysis and

interpretation, ISRO/DOS has set up a system to launch

remote sensing satellites once in three or four years to

maintain the continuity in data collection. The remote

sensing and some of its related institutes are depicted.

Crop weather modeling

Crop model

It is a representation of a crop through

mathematical equations explaining the crops

interaction with both above ground and below ground

environment.

The increase in dry matter of the crop is referred

to as growth. The rate of growth of a healthy crop

depends on the rate at which radiation is intercepted

by foliage and / or on the rate at which water and

nutrients are captured by root systems and therefore

154

on the distribution of water and nutrients in the soil

profile. The crop development is described in terms of

various phenophases through which the crop completes

its lifecycle. That is the progress of the crop from

seeding or primordial initiation to maturity. Finally the

yield of crop stand is expresses as a product of three

components, viz., the period over which dry matter is

accumulated (the length of the growing period), the

mean rate at which dry matter is accumulated and the

fraction of dry matter treated as yield when the crop is

harvested.

It is understood that the crop growth,

development and yield depend upon the mean daily

temperature (DTT), the length of the day and the

amount of solar radiation (PAR) received by the crop.

DTT = Max daily temperature + Min daily temperature

- base temperature

2

Where, DTT = Daily thermal time accumulation.

The time needed for the crop to reach a development

stage depends upon temperature measured above a

base value (DTT) and for photo periodically sensitive

phases such as flowering, the day length above a fixed

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base. In the absence of stress, the harvest index does

not vary much from year to year for a specified

cultivar / variety. Therefore, crop weather modeling is

based on the principles that govern the development of

crop and its growing period based on temperature and /

or day length. They are used to quantify the rate of

crop growth in terms of radiation interception, water

use and nutrient supply which moderate harvest index

when the crops experience stress condition.

The basic information required to be generated for crop

weather modeling includes.

a) Crop phonology in relation to the temperature

and day length

b) Water use by the crop during different

phenophases of crop growth

c) The relationship between radiation interception,

crop water use and total dry matter production

d) Partitioning of dry matter into various plant

components as influenced by water and nutrient

availability, and

e) The effect of weather parameters on biotic

interference to crop growth.

Types of models

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Statistical models

Mechanistic model

Deterministic models

Stochastic models

Dynamic models

Static models

Simulation models

Descriptive models

Explanatory models

Climatic normal

The climatic normals are the average value of 30

years of a particular weather element. The period may

be week, month and year. The crop distribution,

production and productivity depend on the climatic

normals of a place. If the crops are selected for

cultivation based on the optimum climatic requirements

it is likely that the crop production can be maximized.

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WEATHER NORMALS FOR AGRICULTURAL CROPS

Sl. No.

Crops Optimum Temperature °

C

Day length

Rainfall(mm)

Altitude above MSL (m)

Germination

Growth stage

1 Rice Min 10 ° C

22-25 (flowering)20-21(grain formation)20-25(ripening)

1500 <3000

2 Maize 35-44 ° C3 Sorgh

um7-10 25-30 Short

day4 Pearl

millet28-32 400-

7505 Finger

millet500-1000

6 Kodo millet

400-500

7 Wheat 20-22 16-22 250-1800

<3500

8 Barley 12-15 (growth) 30(repr

Long day

400-500

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oduction)

9 Oats 15-25 380-1140

10

Ground nut

27-30 24-27

500-1250

11

Sesame

25-27 Short day

500-650

<1250

12

Castor 20-26 Long day

500-600

<3000

13

Sunflower

20-25 500-700

<2500

14

Rape seed and Mustard

18-25 Long day

300-400

15

Safflower

15-16

25-30 Day neutral

600-900

16

Soybean

15-32

30-33 600-650

1200-2000

17

Pigeon pea

20-30

18

Green gram

15 20-40 Short day

600-1000

19

Black gram

1500

20

Cow pea

12-15

21-35 Short day

600

21

Bengal gram

15-25 600-1000

2 Cotton 18 21-27 Day 500

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2 neutral

23

Jute 27-40 Short day

1500

24

Tobacco

28 25-35 500-1000

25

Sugar cane

24-30 Long day

2000-2500

26

Sugar beet

12-15

22-30 Long day

27

Potato 18-20

18-20

Incidence of pest and disease

Considerable crop losses caused due to pests

and diseases in the humid and sub humid tropics. Many

of the restrictions on productivity and geographical

distribution of plants and animals are imposed by pests

and diseases. The geographical distribution of pests is

mainly based on climatic factors. The climatic

conditions show a gradient from place to place and

there is a related gradient in the abundance of a

particular pest / disease. The periodic or seasonal

nature of incidence and out breaks of several pests and

diseases of many crops can be ascribed to weather

conditions as the triggering factors. These epidemics of

diseases are principally weather dependent, either in 160

terms of local weather conditions being favourable for

growth and development of the casual organisms or the

prevailing winds helping to disseminate airborne

pathogens or spores of diseases such as mildew, rusts,

scabs and blights.

The migration and dispersion of insect pests

depend on the wind speed and direction besides the

nature of air currents. Some plant pathogenic viruses

suitable for the development of these vectors favour

the transmission of such diseases. A surfeit of pests

and diseases, which infest plants are kept in chock by

seasonal fluctuations in atmospheric temperature or

relative humidity and other weather factors. Insect pest

outbreaks occur as a result of congenial weather

conditions, which facilitate their un-interrupted

multiplication. The weather and climate greatly

influence the quantity and quality of food provided by

the host crops to the associated species of pests. The

abundance or otherwise of the pestiferous species is

thus dependent on climatic conditions, indirectly also.

The surface air temperature, relative humidity,

dew fall, sunshine, cloud amount, wind, rainfall and

their pattern and distribution are the primary weather

161

factors influencing the incidence or outbreaks of pests

and diseases of crops. In the humid tropics, the

weather variables namely air temperature, intermittent

rainfall, cloudy weather and dewfall may play a crucial

role in the outbreaks of pests and diseases. The impact

of various weather components on pests and diseases

is experienced in a location and crop specific manner.

Among the major pests associated with crops,

insect, mite and nematode species are of a serious

nature in terms of their abundance and damage

potential. If the occurrence of pest / disease in time and

space can be predicted in advance with reasonable

accuracy on the basis of relevant weather parameters,

appropriate and timely control measures can be

programmed. Appropriate insecticide / fungicide

interventions can certainly reduce the pesticide load in

the environment and the related pollution and health

hazards.

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